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Technical HVAC Terms

Common HVAC Terms

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To terminate a program prematurely (during execution).
absolute pressure (psia)
1) Pressure above a perfect vacuum, usually expressed in pounds per square inch absolute (psia) or in inches of mercury (in. Hg) absolute. 2) The sum of both atmospheric pressure (14.7 psi) at sea level and gauge pressure (psig). Example: If a pneumatic gauge indicates 8 psig, the absolute pressure will be 22.7 psia (8 + 14.7).
absolute zero
1) Temperature at which no heat is present (zero on the Kelvin and Rankin temperature scales, corresponds to 459.7°F and -273°C). 2) The point at which all molecular and electron movement theoretically stops. (This temperature has never been attained.)
The process by which a solid, such as a desiccant, acquires gas or liquid by chemical action.
Refer to alternating current.
The time rate of change of velocity i.e., the derivative of velocity with respect to time.
acceleration, average
The average rate of change of velocity i.e., a change in velocity divided by the time it takes for that change to occur.
access, direct
A method of retrieving information from memory in which the retrieval time is independent of the location in memory Also called random access.
access time
The time it takes a computer to retrieve a bit of information from its memory, also called load time. Or, the time it takes a computer to store information in its memory also called write time. Typical access times are:
  • TFL RAM 60 milliseconds
  • MOS RAM 300 milliseconds
  • Core 500 milliseconds
  • Bubble 3 milliseconds
  • Disk (fixed head) 8 milliseconds
  • Disk (moving head) . .. . 50 milliseconds
  • Floppy disk 100 milliseconds
  • Cassette 10 seconds
  • Tape 10 seconds
A measure of how close an instrument reading is to the actual value.
acoustic coupler
A device (or modem) that allows transmission of digital information over voice- grade telephone lines.
The direction of magnitude change of the output of a controller with respect to the change in the variable that is being sensed.
  • In direct action, the output increases as the variable increases.
  • In reverse action, the output decreases as the variable increases.
1) A device that is mechanically linked to a damper and positions the damper to regulate the flow of air, or that is mounted on a valve and repositions the valve to regulate the flow of steam or water. 2) The part of an electrical switch that is acted upon to cause the switch to change contact connections.
A signal-conditioning device, inserted between any two of the hardware devices designed to accomplish certain applications, which include analog-to-digital interface, impedance matching, isolation sequencing, reversing, or selecting the highest or lowest signal.
A location where particular data or bits of information are stored, such as a memory address or remote point address. This is usually an identification number or identification label.
address selector
A switch on an I/O module used to create a module address.
A term used to describe compression or evaporation that occurs without heat being added or lost. In refrigeration, compression is assumed to be nearly adiabatic.
adjustable differential
A means of changing the difference between the cut-in and cut-out points on a control.
The process by which a solid, such as a desiccant, acquires gas or liquid by capillary action.
air, ambient
Generally speaking, the temperature of the air surrounding an object.
air, re circulated
Return air passed through the conditioner therefore again being supplied to the conditioned space.
air, return
Air returned from the conditioned or refrigerated space.
air, supply
The quantity of air delivered to each or any space in the system, or the total delivered to all spaces in the system.
air changes
A method of expressing the amount of air leakage into or out of a building or room. For one air change per hour, air flow equals the cubic contents of the space expressed in cfm.
air cleaner
A device designed for the purpose of removing airborne impurities.
air conditioning
The process of treating air so as to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of the conditioned space.
air control damper
Used with central fan systems to control the mixture of air admitted to the system. It is installed in the fresh air intake, and the return air and exhaust ducts or in multi zone applications in the hot and cold decks of the air conditioning unit.
air diffuser
A circular, square, or rectangular air distribution outlet, generally located in the ceiling. The typical air diffuser is comprised of deflecting members discharging supply air in various directions and planes. It is arranged to promote mixing of primary air with secondary room air.
air motion relay
Used to sense the air pressure across a coil or fan.
A type of motor that must be mounted in the airstream for proper cooling.
air ventilation
The quantity of supply air required to maintain the desired quality of air within a designated space.
air washer
A water spray system or device for cleaning or humidifying the air.
alarm printer
A printer outfield to log the time of day, point number, and the digital values of a variable that is in an alarm condition.
A set of rules or steps used for solving a problem. Frequently the problem is a mathematical one.
Characteristic of a display that combines alphabetic, numeric, and other characters, such as dollar signs, parentheses, mathematical symbols, etc.
alternating current (ac)
Electric current that reverses its direction at regular intervals.
Fossilized tree sap.
The temperature of the space around a motor. Most motors are designed to operate in an ambient temperature of not more that 40°C (104°F).
A control whose set points are adjusted so that they are not affected by the surrounding environment.
ambient temperature
The temperature of the medium (typically air) that surrounds an object.
American Standard Code for Information Interchange
(ASCII) A standard eight-level code used to simplify and standardize communications between computers and other electronic equipment. It is comprised of seven code bits for information data, and one bit for parity.
American wire gauge (AWG)
The standards adopted in the U.S. for the measurement of wire sizes. (As the wire diameter gets smaller, the number gets larger.)
A device used to measure the current in an electric circuit.
Refers to the maximum current that a wire can carry without overheating.
A unit of measurement of current in an electric circuit. It is based on the number of electrons flowing past a given point (6.25 x 1018 electrons = 1 coulomb) per second.
Any means used to increase voltage, current, or power.
1) The instantaneous value of voltage or current measured in either the positive or negative direction. 2) The variation in distance from rest of a spring or other vibrating body.
A term used to characterize a continuous electrical signal that represents a condition such as pressure, temperature, resistance, current, voltage, or humility.
analog inputs
Inputs that represent physical quantities or process variables, such as distance, pressure, temperature, resistance, current, voltage, or humidity.
analog instrument
An instrument with a reading on a graduated scale
analog-to-digital converter
A device or system that converts an analog signal to a digital signal or code. Abbreviated A-D,AID,or ADC.
An instrument used to measure the velocity of air or other fluid.
annunciator light
A visual indicator of the status of a system usually some form of light or panel of lights that can be used for alarms or to let the operator know the current condition of the system.
The positive terminal of an electronic device, or the P material in a semiconductor.
A small heater in two-position controllers that deliberately causes a false indication to minimize override.
apparent power
The power apparently available for use in an ac circuit that contains a reactive element. Apparent power is the product of voltage times current, expressed in volt-amperes. It must be multiplied by the power factor to obtain the true or actual power available.
A flash caused by the ionizing of a gas or a vapor.
The rotating part of a motor or generator, or the movable part of a relay. The windings in which input current creates a magnetic field that interacts with the main field in a motor. Sometimes used as being identical with rotor.
Refer to American Standard Code for Information Interchange.
To prepare an object language program from a symbolic language program. This is accomplished by inserting machine operation codes for symbolic operation codes.
A computer program that converts mnemonic language to numeric machine language. The usual ratio is 1:1.
Having no regular time relationships. Asynchronous program executions are unpredictable in relationship to time or instruction progression. The opposite of synchronous. asynchronous communication A method of electronic communication in which the data transmission time frame varies between characters.
asynchronous computer
A computer whose operations are not controlled by a master clock.
atmospheric pressure
Pressure exerted by the atmosphere. Standard atmospheric pressure is equal to 14.696 psi or 29.921 in. Hg absolute pressure at sea level.
The smallest particle of an element that stiff maintains its identity.
atomic energy
Energy released when an atom de generates (fission) or combines with others (fusion).
atomic weight
A number indicating the mass or weight of an atom of a substance, compared to that of an atom of hydrogen.
A systemized reduction or lowering of an energy level or signal level.
automatic changeover
Pertains to a thermostat, referring to a system that changes from cooling to heating, and vice versa, without a manual control.
automatic control
A system that reacts to a change in one of its variables by adjusting one of the other variables in order to maintain the balance of the system.
automatic reset
A control that will automatically return to its original position or setting.
Refers to an automatically controlled operation or system, either electronic or mechanical, that takes the place of human observation, effort, and decision.
A transformer in which the primary and secondary are connected together to form one winding.
auxiliary contacts
A set of contacts mechanically attached to and operating in conjunction with a relay, contractor, or actuator motor. They are usually pilot-duty contacts.
auxiliary device
A control module, generally placed between the controller and the actuator, that modifies the controller signal in some manner before the signal reaches the actuator.
  • a relay
  • a switch.
auxiliary potentiometer
A potentiometer that is attached to an actuator and controls another actuator in proportion to the first motor’s action.
auxiliary switch
A switch connected to a valve, motor, or relay that controls a separate circuit.
averaging element
A thermostat sensing element that will react to the average temperature in a duct or controllable variable.
averaging relay
A relay used in applications that operate a final control device or set a controller by the average signal of two controllers.
Refer to American wire gauge.


back pane
How the connections are made to electronic circuit boards.
back up
To make a copy of the data or programs from a hard drive to floppy disks or other storage media.
A surface used for deflecting fluids, usually in the form of a plate or wall.
balancing relay
A pivoted armature that swings between two electromagnetic coils. As the coils change magnetic strength, the relay moves toward the stronger of the two. Also called a mousetrap relay.
ball flapper
An arrangement used in transmitters that varies the exhaust stream by repositioning a small sphere over an opening. This produces output linearity as the sensing element varies its pressure on the sphere.
An auxiliary device used to control voltage and current in gas discharge lamps (such as fluorescent and mercury vapor bulbs).
The magnitude or scope of operation. In communications, the boundaries between two frequencies.
The difference between the highest usable frequency of a device (upper frequency limit) and the lowest usable frequency of the device (lower frequency limit), measured at the half-power points.
An instrument for measuring atmospheric pressure.
An insulator that separates electrical terminals on a relay, compressor, or any other electrical device.
The center semiconductor layer of a junction transistor.
base (number)
1) A type of system that represents numbers by a positional notation. 2) The number of distinct symbols used in a number system. For example, since the decimal number system uses ten symbols (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the base is 10. In the binary number system, the base is 2,because only two symbols (0, 1) are used. Also referred to as radix.
baseline survey
1) IAQ survey to assess air quality when there is no known problem 2) Operational data collected on a refrigeration system prior to retrofitting a refrigerant change.
Two or more cells connected together. The term does not apply to a single cell.
Baud rate
A unit of signaling speed, usually equal to the number of bits or elements per second. Named for Baud (1845 1903), a French engineer and the inventor of the five-channel teletype code. Sometimes used interchangeably with bit rate.
Refer to binary-coded decimal.
A unit of measure for expressing the ratio between two amounts of power (used to refer to the intensity of sound energy). One bel = 10 decibels.
biased start relay
An auxiliary device that accepts a signal from a controller and delays the output until the signal from the controller reaches the set point of the relay.
A strip or coil made of two metals that have different thermal expansion rates, laminated together so that changes in temperature bend the strip or twist the coil.
bimetallic element
A coil or strip of two dissimilar metals used in temperature-sensing devices.
1) Having two elements, parts, or divisions. 2) Having only two possible conditions or values, usually opposed pairs (on/on open/closed, etc.). 3) A number system that uses only two numerals (1 and 0). Also called base 2. Computers utilize a binary code in which the value of each digit’s position is based on the powers of 2, as follows:
128 64 32 16 8 4 2 1
0000 0000 = 0
0000 0001 = 1
0000 0010 = 2
0000 0011 = 3
0000 0100 = 4
0000 1000 = 8
0001 0000 = 16
0010 0000 = 32
0100 0000 = 64
1000 0000 = 128
1111 1111 = 255
binary-coded decimal
(BCD) A mathematical representation of a digit in a decimal number by a four-bit binary number, as follows:
80 40 20 10 8 4 2 1
0000	0000	= 0
0000	0001	= 1
0000	0010	= 2
0000	0011	= 3
0000	0100	= 4
0001	0000	= 10
0010	0000	= 20
0011	0001	= 30
0110	0100	= 64
1001	0110	= 96
1001	1001	= 99
binary digit
One of the two states in the binary system either 1 or 0.
binary number
A number in the base 2 number system. For example, the number 9 in base 10 (or decimal) notation is 1001 in base 2.
binary synchronous communication
A data protocol system developed by IBM whereby information is transmitted twice in order to eliminate errors.
biological contaminants
Microorganisms such as bacteria, viruses, and fungi.
biological sampling
Testing done to discover unwholesome presence of microorganisms.
The single smallest portion of information that a computer can read (abbreviation for binary digit).
bit rate
The speed at which bits travel over a communications channel, expressed in bits per second (bps). Compare with baud rate.
black light
Refer to ultraviolet.
To erase the contents of a cell or range of cells. Both values and formulas can be blanked.
blast heater
A set of heat transfer coils or sections used to heat air that is drawn or forced through it by a fan.
control A pneumatic control arrangement in which the main air to the controller passes through a restrictor. After passing this point, it becomes the branch line pressure.
bleeder resistor
A resistor that is normally soldered in placed, in parallel with a capacitor in order to discharge it.
block diagram
A simplified schematic that uses blocks or symbols to represent the various functions in a system.
blow (throw)
1) In air distribution, the distance that an airstream travels from an outlet to a position at which air motion along the axis reduces to a velocity of 50 ft/mm. 2) For unit heaters, the distance that an airstream travels from a heater without a perceptible rise caused by difference in temperature and loss of velocity.
A closed vessel in which a liquid is heated or vaporized.
boiler horsepower
The equivalent evaporation of 34.5 lb of water per hour at 2 12°F (equal to a heat output of 970.3 x 34.5 = 33,475 Btu per hour).
boiling point
The degree of heat at which a liquid is converted to a vapor by boiling. This point varies for different liquids and for the same liquid at different pressures.
A pipe firmly mounted in a horizontal surface, usually 4-6 inches in diameter and usually cement filled, which protects a piece of equipment from vehicular impact.
boolean algebra
A discipline of mathematics that uses alphabetic symbols to represent variables, and 1 and 0 to represent states. There are three basic logic operations: and, or, and not. Named for George Boole (1815 1886), an English mathematician.
brake horsepower
Actual horsepower developed by a motor, engine, or other machine during operation, as measured by a Prony brake or similar device.
brake winding
The winding in an actuator motor that holds the armature against the brake shoe. This holds the motor in its open position after the limit switch opens. When both brake and motor windings are energized, the motor winding overcomes the brake winding.
branch circuit
Any circuit that comes from the main power lines.
branch lines
The tubing in a pneumatic control system that carries the output signal from the controllers to auxiliary devices or actuators elsewhere in the system.
Generally refers to techniques and/or equipment used to match circuits to each other.
circuit A type of circuit intended to bridge or bypass another circuit, causing a null or no current to be measured.
British thermal unit (Btu)
The amount of heat required to change the temperature of one pound of water one degree Fahrenheit at sea level.
Refer to building-related illness.
The device that carries the current to the armature or commutator of a motor. Usually made of carbon.
Abbreviation for British thermal unit. bubble point 1) The temperature at which a near azeotrope or zerotrope refrigerant will start to vaporize as heat is added. 2) The point at which the last vapor is condensed in a near-azeotrope refrigerant as heat is removed change of state. 3) The process of a gas changing to a liquid, a liquid changing to a gas, a liquid changing to a solid, or a solid changing to a liquid. Changes of state occur during evaporation, boiling, condensation, fusion, melting, solidification, and sublimation.
buck-boost transformer
Refer to autotransformer.
bucket trap (inverted)
A float trap with an open float. The float or bucket is open at the bottom. When the air or steam in the bucket has been replaced by condensate, the bucket loses its buoyancy When it sinks, it opens a valve to permit condensate to be pushed into the return.
bucket trap (open)
The bucket (float) is open at the top. Water surrounding the bucket keeps it floating, with the pin pressed against its seat. Condensate from the system drains into the bucket. When enough condensate has drained into the bucket so that it loses its buoyancy it sinks and pulls the pin off its seat. Steam pressure forces the condensate out of the trap.
A circuit or system located between two pieces of equipment or between two data storage units. Information can be held in the buffer until the computer is ready to use it.
A program or hardware malfunction. building-related illness (BR1) Illness that is clearly diagnosed and is clearly related to the building. bulb The name given to the sensing element of a controller. Also applied to a device that emits light. burn A permanent set of instructions entered into a PROM or EPROM.
burn in
A form of testing new equipment or components to verify their stability and to detect early failures.
A set of parallel conductors that connects pieces of hardware in any computer system.
bus, bidirectional
A set of parallel conductors that allows for the transmission of data in either direction between pieces of hardware in any computer system.
bus bar
A primary power distribution point. Sometimes used to denote small copper or aluminum conductors. See also strap.
A pipe or duct controlled by a valve or damper, used to carry air or a fluid around a device in the system.
Usually, eight bits of information. Memory is normally measured in the number of bytes of information that it can store.


cad cell
A cadmium sulfide cell that changes resistance with light. As light falls on the cell, the resistance decreases. Used in conjunction with primary oil controls.
A precision correction or adjustment to the scale of a control, instrument, or device in order to bring it into compliance with certain standards for example, adjusting a controller’s set point to match the ambient temperature that it is controlling.
calibration point
The output pressure of a controller when the set point and control point are equal. Example:
For a controller with an output of 3 to 13 psig, the calibration point normally would be 8 psig (the midpoint of the range).
The amount of heat required to change the temperature of one gram of water one degree Celsius.
1) A device used for measuring heat quantities, such as machine capacity; heat of combustion, specific heat, vital heat, heat leakage, etc. 2) A device used for measuring quality (or moisture content) of steam or other vapor.
capacitive reactance
Opposition by a capacitor to the flow of current in an ac circuit, measured in ohms.
An electrical device used for storing a charge of electrons.
capacitor-start motor
A type of single-phase, ac induction motor in which a start winding and a capacitor are placed in series to start the motor.
capacitor-start-run (CSR)
A motor that uses both a start and run capacitor to increase starting torque and running efficiency.
capacitor-start, capacitor-run (CSCR)
Refer to capacitor-start-run.
capacitor-start, induction-run (CSIR)
A type of motor that uses a start winding and start capacitor only during starting.
The usable output of a system or system component in which only losses occurring in the system or component are charged against it.
capacity Index (CV factor)
The quantity of water, measured in gallons per minute, at 60°F that will flow through a given valve with a pressure drop of 1 psig. See also flow coefficient.
capillary tube
1) A small-diameter tube used as a metering device in a refrigeration or air conditioning system. 2) The small tubing that transmits the temperature to a controller from a sensing element.
carbon dioxide
(C0 Odorless gas given off in the processes of breathing and in combustion. Too much carbon dioxide reduces the amount of oxygen in the air.
carbon monoxide (CO)
Deadly gas given off by gasoline vehicles and by propane, natural gas, or kerosene heaters.
card cage
An enclosure designed with slots for holding printed circuit boards.
An analog signal at a fixed amplitude and frequency that combines with an information signal in a modulation process to produce a transmittable signal.
cascade control
A control system in which the controls are linked in a chain-like fashion, with the output of one stage feeding the input of the next stage. Sometimes called piggy back control.
cascade refrigeration
A compound refrigeration system in which the evaporator of the high stage removes heat from the condenser of the low stage. Usually used for temperatures below -40°.
The negative terminal of a device, and the N material of a semiconductor.
cathode ray tube (CRT)
A display terminal, similar to a television set. cavitation The formation and collapse of vapor pockets (bubbles) in a liquid, due to the increase and decrease of pressure as the liquid flows through a restriction.
Refer to central control unit.
The unit of storage in a spreadsheet program is the space at the intersection of a row and a column.
cell reference
The inclusion of a cell’s value in another cell or cells, or in a formula, by the inclusion of the referenced cell’s coordinates in the referencing cell, cells, or formulas.
Formerly Centigrade, a thermometric scale in which 00 is the freezing point and 1000 is the boiling point of water at sea level.
Refer to counter electromotive force.
central control unit (CCU)
In an automated control system, the central site at which all of the master controls and peripheral equipment are located.
central processing unit (CPU)
The section of a computer that controls all of the information operations and control operations.
centrifugal starting switch
A switch mounted on the shaft of a motor that switches the start winding in and out of the circuit, depending on the rotor speed.
Refer to cubic feet per minute.
change of air
The introduction of new, cleansed, or re circulated air to conditioned space, measured by the number of complete changes per unit of time.
change of state
A change from one phase, such as solid, liquid, or gas, to another.
The process of changing from a heating to an air conditioning system, or vice versa.
A frequency band wide enough for one- way communication from a radio or television station, or from a microwave source. An electrical transmission path between two or more stations in communications. Also, a termination furnished by wire or radio or both.
A numeral from 0 to 9, a letter from A to Z, a mathematical symbol, a punctuation mark, or any other symbol that a CRT or printer can process as output or input. Usually a numerical representation in a computer.
Refer to relay contact bounce. chemical compound A substance whose molecule consists of atoms of two or more elements. Its unit is the molecule.
chemical energy
The energy released by a molecule of a substance in breaking up into the separate atoms of the elements comprising it.
chemical symbol
A combination of letters and numbers indicating the identity and number of the atoms comprising the molecule of a substance. For example, the water molecule symbol H shows that it consists of two atoms of hydrogen and one atom of oxygen.
chimney effect
1) The tendency of air or gas in a duct or other vertical passage to rise when heated, due to its lower density compared with that of the surrounding air or gas. 2) In buildings, the tendency toward displacement (caused by the difference in temperature) of internal heated air by unheated outside air due to the difference in density between outside and inside air.
Refer to integrated circuit.
choke coil
A coil of wire of low resistance and high impedance to block the flow in an alternating current circuit and pass DC current. Used with some shaded-pole motors for speed control.
The complete path necessary for an electric current to flow. It must consist of a source, a path, and a load.
circuit breaker
An electromagnetic or thermal device that interrupts current flowing through a circuit.
circular mu
An area equal to that of a circle with a diameter of 0.00 1 inch. It is used in measuring the cross section of a wire.
Usually a state denoting zero or blank. clearance volume That portion of a cylinder between the top of the piston and valve plate at the end of the compression stroke.
Any device that controls a circuit by using time e.g., the oscillator in a computer.
closed loop
An automatic control system that incorporate a feedback signal to correct any errors between the actual value and the desired value.
The maximum allowable pressure drop to which a valve may be subjected while fully closed.
close-off rating
The maximum allowable pressure drop (inlet to outlet) that a valve will tolerate when fully closed. The power available from the actuator usually determines this value.
Refer to complementary metal-oxide semiconductor.
Refer to carbon dioxide. CO Refer to carbon monoxide.
coaxial cable
A cable or transmission line that consists of two or more conductors, each separated by a dielectric material, with a common axis.
A system of symbols used for communication, or a system of symbols that represents data.
code, binary
A computer language that uses only ones and zeros (base 2).
code, hexadecimal
A computer language that incorporates the numerals 0 through 9 and the letters A through F to represent a group or groups of four binary digits (base 8).
coefficient of cubical expansion
The increase or decrease in the volume of a material for each degree change in temperature.
coefficient of linear expansion
The increase or decrease in one dimension of a material for each degree change in temperature.
coefficient of performance (COP)
The ratio of the refrigerating capacity of a system to the motor input energy both expressed in Btu. It is obtained by dividing the wattage input to the electric motor of a condensing unit, multiplied by 3.4 12, into the net refrigerating capacity of the evaporator, expressed in Btu per hour.
The end portion of a junction transistor that is reverse-biased with respect to the base.
comfort chart
A chart showing effective temperatures with dry-bulb temperatures and humidities by which the effects of various air conditions on the comfort of human occupants may be compared.
comfort line
A line on the comfort chart showing the relationship between the effective temperature and the percentage of adults feeling comfortable.
comfort range
Range of conditions, especially temperature and humidity that provide a comfortable indoor environment.
A signal or pulse to start, stop, or continue an operation in a computer. A synonym for instruction.
command file
A file created and stored on a disk that contains a set of executable ONcalc commands. You call a command file by its name, and the file then executes the commands.
A terminal, connection, or wire that is shared by other parts of an electrical network.
The copper segments on a motor’s rotor, connected electrically through brushes.
A device used on some telephone channels to compress or expand the speed, range, and volume of incoming and outgoing signals.
compensated control
A controller whose set points are not affected by the ambient temperature.
A program that a computer uses to interpret high-level language into machine- oriented language.
complementary metal-oxide semiconductor (CMOS)
An integrated circuit with low power requirements and very high noise immunity.
compression ratio
A number found by dividing head pressure by suction pressure, both expressed in psia (add 14.7 to the gauge pressure of each).
A device that provides the pressure increase in a refrigeration or air conditioning system.
compressor displacement
The internal volume of the cylinders of a compressor multiplied by its rpm. It indicates the cfm that a compressor of 100% volumetric efficiency would pump. computer A machine designed to receive data (input), process the data at high speeds, and then produce the results (output).
computer, analog
A computer that designates variables by physical analogies. It resolves problems by translating conditions such as flow, pressure, temperature, voltage, or current quantities into electric circuits being investigated.
computer, digital
A computer that processes data by using discrete number systems— e.g., binary hexadecimal, etc.
The liquid formed by condensation of a vapor. In steam heating, water condensed from steam. In air conditioning, water extracted from air, as by condensation on the cooling coil of a refrigeration machine.
The process of changing a vapor into a liquid by the extraction of heat (cooling the vapor below its dew point). Condensation of steam or water vapor is effected in either steam condensers, or coils, or in dehumidifying coils, and the resulting water is called condensate.
A device that reduces a vapor to a liquid. condenser heat rejection The total amount of heat removed from a system by the condenser.
condenser heat rejection factor
The amount of heat absorbed in the evaporator as a percent of the condenser heat rejection.
condensing pressure
The saturation pressure corresponding to the condensing temperature.
condensing temperature
The temperature at which a gas changes to a liquid in the condenser.
1) Same as conductivity (see below), but without regard to thickness. The material may consist of several layers of different materials. Also called the C factor. 2) The ability of a conductor to carry current, measured in mhos or siemens.
conduction of heat
Heat flow through a material by contact between particles.
The rate of heat flow, in Btu per hour, through one square foot of material, one inch thick, with a temperature difference of one degree Fahrenheit. Referred to as the K factor.
conductivity, electrical
The property of a material that permits an electric current to pass through it.
conductivity, thermal
The rate of heat flow through a material under stable conditions that is, when the temperature difference between the two ends of a heat conductor remains constant.
Any material suitable for carrying an electric current.
conductor, thermal
A material that readily transmits heat by means of conduction.
conservation of energy
The law that states that energy may neither be created nor destroyed only transformed from one form to another.
constant cut-In
A control whose cut-out setting can be adjusted, but whose cut-in remains the same. See also ow-cycle defrost control.
constant cut-out
A control whose cut-in setting can be adjusted, but whose cut-out remains the same. Used in two-temperature refrigerators and defrost systems.
constant-volume control
An air system in which the total quantity of air flow remains the same. The load is matched by changing the temperature of the air delivered.
contact rating
The electric power or current- handling cap ability of switch or relay contacts. Usually measured in amperes, volts, or horsepower, either resistive or inductive.
contacts, in
Applies to a relay when the contacts are pulled in so, when the contacts are closed.
contacts, out
Applies to a relay when the contacts are out also, when the contacts are open.
Cells grouped together with no intervening items. A contiguous range includes all cells within the boundaries of that range.
Any device used for the regulation of a system or component in normal operation, manual or automatic.
control agent
The medium manipulated by a control system to cause a change in the controlled medium for example, the steam in a heating system or the refrigerant in a cooling system.
control hunt
A condition of instability in a control system in which the controller over-reacts in either direction. Sometimes simply called hunting.
control loop
A series of components that make up a control system. If feedback is incorporated, it is called a closed-loop system if not, it is called an open-loop system.
control medium
The agent, manipulated by the controlled device, that serves as the transport system of heating or cooling. Examples: chilled water, hot water, steam, etc. control point The middle of the range of values that a control tries to maintain under given conditions.
control range
The actual operating range of a control.
control unit
Part of the central processing unit (CPU). It directs the sequence of operations, initiates commands to the computer circuits, and executes instructions.
control, direct digital (DDC)
A control loop in which data are updated periodically through a set of given control algorithms.
control, distributed
The distribution of data and operational control, usually through computers, by networking. Each controller maintains its own independent operation, even if the communication with the rest of the network is lost.
control, floating
A change in a controlled variable that causes a control to make a contact, which in turn causes an operator to run. The operator will not stop until the variable returns to the control set point on the controller.
control, integral
A control method (or algorithm) in which the final control element is moved in a corrective direction at a rate proportional to the error (deviation) of the controlled variable, until the controller is satisfied.
control, proportional/integral/derivative (PID)
A control algorithm that enhances the PT control algorithm by adding a component that is proportional to the rate of change (derivative) of the deviation (error) of the controlled variable. This serves to anticipate future errors under current conditions. Also referred to as three-mode control. controlled device An apparatus that receives the signal from a controller and positions the damper or valve to match the capacity to the load. Examples: a motorized damper or valve.
controlled medium
The substance (usually air, water, steam, or refrigerant) whose characteristics (such as temperature, pressure, flow rate, volume, concentration, etc.) are being controlled.
controlled space
The volume of the controlled medium, such as a room to be heated or cooled.
controlled variable
The quantity, characteristic, or condition of the controlled medium that is to be measured for example, temperature, pressure, etc.
A device that receives a signal from a remote sensor or from its own integral sensor and then produces an output signal to a controlled device. Examples: thermostats and receiver controllers.
The transfer of heat by the circulation of fluids due to differences in temperature and density.
The means of convection. In heat transfer, a surface designed to transfer its heat to a surrounding fluid or air, largely or wholly by convection. The heated fluid may be removed mechanically or by gravity Such a surface may or may not be enclosed or concealed. conversational A method of communication in which a human communicates directly with the CPU, by issuing commands and deriving responses to the operator.
A device or circuit that changes the information or voltage in a system e.g., ac to dc voltage, analog to digital, etc.
cooling leg
A length of un insulated pipe through which the condensate flows to a trap. It has sufficient cooling surface to permit the condensate to dissipate enough heat to prevent flashing when the trap opens. In the case of a thermostatic trap, a cooling leg may be necessary to permit the condensate to drop a sufficient amount in temperature to permit the trap to open.
Refer to coefficient of performance.
The two numbers that uniquely identify the column and row of a cell on the worksheet.
To duplicate a program or data without changing the original information.
A magnetic material that affords a path for magnetic flux.
A program that is located permanently in the main memory of a computer.
A luminous discharge around the surface of a conductor caused by the ionization of the surrounding air by high voltages.
correctIve action
Action initiated in response to a deviation of a signal from the desired value, and resulting in a change in the manipulated variable.
A measure of the quantity of electricity. One coulomb is equal to the charge of 6.28 x 1018 electrons. See also joule.
A device for storing a number and increasing or decreasing it. Also, a register or storage location for recording the number of occurrences in an event.
counter electromotive force (CEMF)
The voltage generated within a coil by a moving magnetic field cutting across the coil itself. Counter electromotive force voltage is in opposition (counter) to the moving field that created it. Also called back electromotive force.
Refer to central processing unit.
An unplanned system shutdown caused by a software, hardware, or power failure.
critical pressure drop
1) Fluid flow through a valve increases with increased pressure drop until a critical point is reached. Any drop in excess of this value will cause noise and wear. 2) The maximum pressure drop allowable in order to minimize noise, expressed in absolute pressure at the inlet port of a valve.
cross ambient fill
A vapor pressure element sufficiently large to ensure that there is liquid in the bulb, regardless of whether the bulb is warmer or cooler than the ambient temperature.
The unwanted transfer of a signal from one path to another e.g., between data and/or communication transmission lines.
Refer to cathode ray tube.
Refer to capacitor-start, capacitor-run.
Refer to capacitor-start-run.
Refer to capacitor-start, induction-run.
cubic feet per minute (cfm)
A unit of measure used to quantify air flow.
cubic Inches per minute (in
A unit of measure used to quantify air flow.
The movement of electrons past a reference point. The passage of electrons through a conductor, measured in amperes. The symbol for current is I.
current limiter
A protective device for a circuit, usually a fuse or circuit breaker.
An indicator on the face of a CRT or visual display to show where the next character will be placed.
cut-In setting
The point at which the contacts on a control will close.
cut-out setting
The point at which the contacts on a control will open.
One complete alternation of electric current, consisting of one positive half-cycle and one negative half-cycle. See also frequency. Also, a repeated sequence of events or operations, or the complete course of operation of a system.
cycle time
The least amount of time allowed between subsequent accesses to the same memory.
Refer to hunting.
1) A device that converts fluid power into linear mechanical force and motion. 2) A closed container for fluids.


Refer to direct-acting.
A device used to vary the volume of air passing through an air outlet, inlet, or duct.
damper, opposed-blade
Alternate blades rotate in opposite directions, which provides an equal flow characteristic.
damper, parallel-blade
All blades rotate in the same direction, which provides a fairly linear air flow.
A term used to denote all numbers, letters, symbols, and other information that can be stored, altered, or transmitted. Generally pertains to the information stored and processed by a computer.
data base
A sizable amount of information, stored in a computer, that can be retrieved with special instructions as to organizing, sorting, or deleting.
data file
A raw file of data in a format that can be used and understood by a computer program.
data link
A channel of information that connects any input, output, or any other peripheral device to the data processing apparatus.
day clock
A type of computer clock that actually keeps the time of day and the date.
day-night thermostat
A device that consists of two thermostats operating at different times or temperatures. Now done with micro-electronics.
Refer to decibel.
A unit of measurement used in the telephone industry to denote power level. Based on an impedance of 600 f and a frequency of 1,004 Hz. (0 dBm is equal to 1 mV at 1,004 Hz terminated by a 600- impedance.) dc Refer to direct current. DDC Refer to direct digital control.
1) Deadband is an area in which no action is taking place. In thermostats, it is the area in which there is no control of the unit. 2) In a control system, the range of values through which the controlled variable can be altered without initiating a corrective response.
dead man timer
A method used to detect a computer malfunction that requires a corrective action.
To identify; locate, and remove faults from a computer program or routine.
debug program
A utility program that provides the capability to inspect, control, and debug other programs during executive command.
decibel (db)
A unit of measure for expressing sound or power intensity levels. The smallest change in sound intensity that the normal human ear can detect. See also bel. decimal A number system in which the numerals 0 through 9 are used (base 10). Each number is called a digit. The system has a radix of 10.
decimal number
A figure written in the base 10 numbering system. When reading the number from left to right, each digit is perceived to be multiplied by a progressively higher power of 10. The digits are referred to as the ones, the tens, the hundreds positions, and so on. decision In computer terminology, decisions are yes or no answers to simple questions that have been refined until they can have only one of two possible answers. Computers can make these decisions so rapidly that a complex logic routine appears to be done instantly.
dedicated line carrier
A form of two communications between input and output devices and controllers that requires only a two- wire communications link.
To terminate current flow to a device. default An automatic response to a command or other user action in the absence of the user specifying what the response should be. default value The value assumed by a program, unless another value is stipulated by the user.
Refers to a reduced level of service at which a system is operated. dehumidifying Removing moisture from the air.
degree day
A unit, based on temperature difference and time, used in estimating fuel consumption and specifying the nominal heating load of a building in winter. For any one day, when the mean temperature is less than 65°F, there exist as many degree days as there are Fahrenheit degrees difference in temperature between the mean temperature for the day and 65°F.
To erase the contents of a cell, group of cells, or an entire column or row. The worksheet automatically adjusts cell references on deletions.
A character that separates items in a list. The default delimiter for a ONcaic argument list is the comma. The default delimiter for data exchange ivies is the character #. delta Prefix meaning three. Derived from the Greek letter delta (s), which looks like a triangle. It refers to the change in or the difference between in such terms as delta T, applied to temperature ( = final temperature minus initial temperature).
delta transformer
A three-phase transformer whose windings are wired in a delta (or triangular) pattern.
The power (measured in kVA) drawn from an electrical utility averaged over a suitable period of time usually 15 or 30 minutes.
To remove a carrier or RF frequency from a primary signal.
The weight or mass of one cubic foot of a gas, liquid, or solid, normally expressed in pounds per cubic foot.
A material that has a strong affinity for water, and therefore used for removing moisture from the refrigerant in a system, either by absorption or adsorption. design conditions Space temperature conditions that require the full heating or cooling requirements of a system.
desired value
The point at which the value of the controlled medium is to be maintained.
The difference between the controlled condition and the set point, or between a maximum (or minimum) and an average value. See also droop.
A computer peripheral or an electronic component.
dew point
The temperature at which the condensation of water vapor appears.
dew point (refrigerant)
The temperature at which a zeotrope or near-azeotrope refrigerant will start to condense as heat is removed. Also, the point at which the last liquid refrigerant is evaporated as heat is added to a near-azeotrope refrigerant. dew point temperature The air temperature at which the condensation of water vapor begins as the air is cooled.
A special routine designed to locate a malfunction (and identify its cause) in a computer or other device.
The insulating material that separates the electrodes of a capacitor. These materials consist of aiz mica, plastic, wax-impregnated paper, and ceramic.
dielectric strength
The ability of an insulator to withstand a potential difference without breaking down (usually expressed in terms of voltage).
In a temperature or pressure control system, the difference in temperature or pressure between the opening and closing of the contacts.
differential pressure control
A system in which two pressure sensors transmit their respective signals to a controller. The controller, in turn, produces an output to the controlled device that will vary in accordance with the difference of the two sensed pressures.
A character used to represent an integer smaller than the base radix e.g., 0 to 9 in decimal notation, 0 or 1 in binary notation digital Pertains to having two states, such as ON and o open and closed, 0 and 1. See also binary.
digital device
Any piece of equipment that employs digital methods in its operation.
dlgltal-to-anaiog converter
A device that accepts a digital input signal and converts it to an analog output, usually voltage or current. Abbreviated D-A, D/A, or DAC.
To change an analog signal or measurement into digital form.
Refer to distributed input/output. diode A two-element device that allows current to flow in one direction only. When the diode is reversed, current will not flow. It can be either a vacuum tube or a solid-state device.
Refer to dual in-line package.
direct-acting (DA)
An instrument in which the output signal changes in the, same direction as the controlled variable changes i.e., the response increases as the change in the variable increases.
direct current (dc)
An electric current that flows in one direction only, such as a battery delivers. The current flows from the negative to the positive terminal. direct digital control (DDC) A control loop in which data are updated periodically through a set of given control algorithms.
direct reset
In multiple-input (usually two-input) applications, a control technique in which an increase at the second (open-loop) sensor causes the controller set point to be increased. distributed input/output (DiO) An I/O technique used by Computer Automation, Inc. that multiplexes the primary computer bus.
distributed processing
A concept whereby decision making is shared by multiple computers and peripherals.
diverting valve
A three-way valve (with one inlet and two outlets) that can direct full flow to either of the two outlets or proportion the flow between the two outlets.
domestic hot water
Hot water used for purposes other than for house heating, such as for laundering, dishwashing, bathing, etc.
DOP test
A test used for evaluating the performance of very high-efficiency filters.
A switching arrangement in which two separate contacts open and close simultaneously.
double-seated valve
Fluid pressure is introduced between two seats, enabling the valve to close off against high pressure.
A switching arrangement in which contacts make in one position and simultaneously break in the other.
down-feed one-pipe riser (steam)
A pipe that carries steam downward to the heating units into which the condensate from the heating units drains.
The transfer of electronic information from a central processing unit to ancillary devices, such as floppy disks.
The period of lime when equipment is malfunctioning or off-line. Also, production time lost when equipment is being repaired.
A current of air. When referring to the pressure difference that causes a current of air or gases to flow through a flue, chimney, heater, or space, or to a localized effect caused by one or more factors of high air velocity low ambient temperature, or direction of air flow, whereby more heat is withdrawn from a person’s skin than is normally dissipated.
A term given to denote the difference between the operating point and the set point of a control.
A pipe or steam trap and a pipe considered as a unit, which conducts condensation from the steam side of a piping system to the water or return side of the system.
A term used to demonstrate the difference between the set point and the control point of a control.
The vertical distance that the lower edge of a horizontally projected airstream falls or rises as the combined result of density effect of cool or warm air and the vertical expansion of the air path resulting from entrainment.
The value of current or voltage at which an energized relay will be de-energized.
dry-bulb temperature
The temperature indicated by an ordinary thermometer, it indicates the sensible heat of air and water vapor mixture.
dual in-line package (DIP)
A packaging technique for integrated circuits that incorporates two parallel rows of connecting pins.
dual thermostat
A two-temperature thermostat, equivalent to two separate thermostats under one cover, which has two set points and two different outputs.
A passageway made of sheet metal or other suitable material, not necessarily leak-tight, used for conveying air or other gas at low pressure.
To read out the entire contents of a computer’s memory into another computer or a printer.
dust spot test
A test used for medium- and high- efficiency filters. Also called atmospheric dust weight arrestance rating.
dust weight test
A test used for low-efficiency lifters. Also called dust weight arrestance rating.
duty cycle
An energy management program that lowers the consumption of electric power by cycling the equipment so that minimum operating time is used for comfort conditions.
duty cycle
(motor) The relationship between the time during which a motor is operating and the time during which it is at rest. There are two terms used in the HVAC/R industry continuous cycle, which refers to a motor that operates for periods of more than one hour at a time, and intermittent cycle, which refers to a motor that operates for periods of 15 to 30 minutes at a time.


Electrically alterable read-only memory; a type of memory that combines the characteristics of RAM and ROM. It is non-volatile (like ROM), but can be written into the processor (like RAM). EAROM has a substantially longer writing time (currently about 2 microseconds versus 400 nanoseconds), as well as a limited number of writes (about 1,000,000) before the chip can no longer be programmed.
A control system that provides outside air if possible when cooling is required. Also can refer to a chilled water system that substitutes condenser water for chilled water during periods of low ambient temperatures.
economizer control
A system of ventilation control used to ensure proper mixed air temperature, and to get free cooling.
eddy current
Induced circulating currents in a conducting material that are caused by varying the magnetic field.
To change, add or delete information from a file or program, text files, or data bases.
A utility program that performs editing procedures on programs and data automatically.
Refer to electronic data processing. effective area The net area of an outlet or inlet through which air can pass. It is equal to the free area of the device times the coefficient of discharge.
electric demand
limiting The controlling of the instantaneous draw of power from a utility’s circuits.
electrical alterable react-only memory
Refer to EAROM.
electrical degree
One 360th (½6o) of a cycle in a sine wave. One complete cycle equals 360 electrical degrees.
electrical rating
The electrical power-handling capacity of a switch or relay.
electric-pneumatic switch (EP)
An electrically operated air flow switch with normally closed and normally open inputs that lead to a common output. Also known as a solenoid air valve.
In a semiconductor, the direct transfer of current into light e.g., an LED.
The chemical paste used in some capacitors and the chemical that causes the current action in a battery.
A magnet formed by wrapping wire around a metal core, and then applying an electric current to the coil of wire.
electromotive force (EMF)
The difference of potential that forces electrons through a resistance, measured in volts.
One of the fundamental particles of an atom that is negatively charged.
electronic data processing (EDP)
The transformation of raw data into useful data by means of electronic equipment.
1) A substance that cannot be separated into simpler substances by chemical means, and does not lose its identity through chemical action. Its unit is the atom. 2) Any device in a system, such as a resistor, capacitor, or transformer. Also, the basic component of an atom that cannot be broken down any further and still retain its identity.
element, bimetallic
An element formed of two metals that have different coefficients of thermal expansion, such as are used in temperature- indicating and controlling devices.
element, electric heating
A unit assembly consisting of resistance wire, insulated supports, and terminals for connecting the resistance wire to electric power.
Refer to electromotive force.
The end portion of a junction transistor, normally forward-biased with respect to the base.
To place into a coded format.
The application of some form of energy to a device i.e., to apply a voltage.
The ability to do work. The physical capacity to move something through a distance against a resistance.
Heat content, usually based on 40°F, expressed in Btu per pound.
The induced flow of room air by the primary air from an outlet, creating a mixed air path (commonly called secondary air motion).
The amount of energy that is unavailable for work during a natural process i.e., energy not practically usable for a given application. A change in entropy is the summation of the changes in enthalpy/absolute temperature for a given process.
Refer to electric-pneumatic switch.
Environmental Protection Agency.
Refer to erasable programmable read-only memo!)’.
equal percentage valve
A flow characteristic through a valve in which the seat-plug configuration is such that each equal increment of stroke movement produces a change in flow equal to the continuously increasing increase over the linear increase.
erasable programmable read-only memory
Erasable, programmable read only memory, a memory capable of being electrically programmed by external means to the computer. It then functions as a read only memory when used by the computer. It can be erased by ultra-violet light, and then re-programmed.
Environmental tobacco smoke.
To run a program or perform an instruction in a computer.
In computer terminology; a set of commands, instructions, or routines that controls the computer’s resources. Also called the operating system.
Leakage of air out of a building.


face-and-bypass damper system
A heating system in which the air can be bypassed around a heating or cooling coil (face) by the use of opposed operating dampers.
factor of safety
In pressure vessels, the ratio of ultimate stress to design working stress.
Capable of compensating automatically for a failure by returning to a default position e.g., a damper will close if there is a loss of power.
fan, centrifugal
A fan rotor or wheel within a scroll- type housing, including the driving mechanism supports for either belt drive or direct connection.
fan, propeller
A propeller or disk-type wheel within a mounting ring or plate, including the driving mechanism supports for either belt drive or direct connection.
fan, vaneaxlai
A disk-type wheel within a cylinder, a set of air guide vanes located either before or after the wheel, and the driving mechanism supports for either belt drive or direct connection.
The basic unit of capacitance. A capacitor has a capacitance of one farad when a potential difference of one volt across it produces a charge of one coulomb.
A design feature of proportional control in which the controller receives a signal from the controlled device, thereby telling the controller what position the controlled device has assumed.
feet per minute (ft/mm)
A unit of measure used to quantify the velocity of air flow.
The space containing electric or magnetic lines of force.
field (electric)
The region around (or within) a current-carrying conductor in which an electric force can act on a charge.
field (magnetic)
The region in which the magnetic forces (north and south polarity) created by a magnet can act on materials.
field (motor)
The stationary part of a motor that furnishes the magnetic field surrounding the armature.
1) In electronic circuits, a device that blocks an unwanted signal e.g., a power supply filter removes excess noise. 2) A device used to catch and remove impurities from a fluid. fin An extended surface designed to increase the heat transfer area e.g., metal sheets attached to tubes.
finish point
The pressure necessary to compress the spring of an actuator completely and cause the actuator to complete its stroke. Example: For an actuator with a spring range of 5 to 10 psi, the 10 is the finish point. See also start point.
A high-temperature safety control, designed to shut down the system when it detects a high temperature in the ductwork
Any computer program that resides in a ROM, PROM, or EPROM.
fixed differential
The difference between the cut-in and the cut-out. It is factory-set and cannot be adjusted.
fixed duty cycle
A fixed number of on minutes for which a system will operate each hour.
fixed setting
A factory-set variable for which no user adjustment is available.
Refer to full-load amperage.
flame safeguard control
A control that proves that there is a flame for the burner control to operate.
float and thermostatic trap
A float trap with a thermostatic element for permitting the escape of air into the return line.
float trap
A steam trap operated by a float. When enough condensate has drained (by gravity) into the trap body, the float is lifted. This in turn lifts the pin off its seat and permits the condensate to flow into the return until the float has been sufficiently lowered to close the port. Temperature does not affect the operation of a float trap. floating action In a control system, continuous action in which the rate of change of the output is a predetermined function of the input. The movement of a controller toward its open or closed position.
flow chart
A diagram that depicts all of the steps of a process, program, or system in a logical sequence.
flow coefficient
The number of gpm of water (at 60°F) that will flow through a fully opened valve with a 1-psi drop across the valve. Its symbol is CV. See also capacity index.
flow rate
The amount of fluid flowing past a given point per unit of time. Typical units are gpm for water, lb/hr for steam, and cfm for air.
1) The field in which magnetic lines of force are found. 2) A substance used to keep oxides from forming during a soldering or a brazing process.
foot candle
A unit of illumination equal to the light falling on an area of one square foot with the uniformity of one lumen.
The push or pull on a body that tends to change its state of motion.
The graphic layout of a cell e.g., with a percent sign with decimal places shown, as a graphic symbol to create bar graphs.
The principal unit in creating a model. A formula is entered in a cell, and can be either a cell’s coordinates, a function, an arithmetic expression, or any combination of these three.
FORmula TRANslator, a type of high-level language used in computers to solve scientific and engineering problems.
fractional horsepower
A horsepower value of less than one.
Refers to a system developed by NEMA for standardizing the mounting of electric motors according to various dimensions.
free electrons
Electrons that are loosely held in orbit around an atom.
freeze stat
A type of control used to prevent the freeze-up of coils and other devices.
The number of vibrations or cycles in a given period of time e.g., cycles per second (hertz), revolutions per minutes (rpm), etc.
full-load amperes (FLA)
The term used to indicate the current drawn by a motor running at its full rated horsepower and voltage.
full-wave rectifier
A circuit that utilizes both the positive and negative cycles of an alternating current to produce a direct current.
A predefined calculation or constant. function key A button or key on a computer that gives the machine a specific command to perform a specialized job.
A complete heating unit for transferring heat from fuel being burned to the air supplied to a heating system.
A non-reusable device that breaks a circuit when the current exceeds a certain level.
Melting, the change of state from a solid to a liquid or vice versa.


Refer to gallons per minute. gain Any increase in the strength of a signal, calculated as the ratio of the output signal to the input signal.
gallons per minute (gpm)
A unit of measure used to quantify liquid flow.
An instrument that measures small direct currents.
gas constant
Coefficient R in the universal gas formula PV= RT, determined by dividing 1,546 by the molecular weight of the gas.
gas, Inert
A gas that causes no chemical reaction and undergoes no change of state. Krypton, argon, boron, helium, xenon, and neon are some of the inert gases. Also called noble gases.
A circuit that may have many inputs but just one output. The output usually remains unchanged until certain input conditions are met. These circuits perform Boolean logic operations.
An instrument used for measuring pressure or liquid level. Also, an arbitrary scale of measurement for sheet metal thickness, wire and drill diameters, etc.
gauge, glass
A device used for showing a liquid level. gauge pressure (pslg) 1) The pressure read on a gauge that has been adjusted to read zero under atmospheric pressure at sea level. 2) The amount of pressure above atmospheric pressure.
A rotating electric machine that converts mechanical energy into electrical energy This may be driven by different means such as a gas engine, water or steam.
A crystalline semiconductor material, used to make transistors and diodes. glide The difference in temperature between the dew point and the bubble point for a zeotrope or near-azeotrope refrigerant.
global settings
Specifications affecting every cell on a worksheet e.g., format, recalculation order, recalculation mode, and column width. The G command resets any global setting. The format command can locally override global and default settings.
gradual switches
Manual switches that proportion the controlled condition in reference to the position of the switch. In pneumatics, a device for adjusting the air pressure from zero to full line pressure.
gravity, specific
The density of a material compared to the density of a standard, such as water.
A louvered or perforated covering for an air passage opening, which can be located in the sidewall, ceiling, or floor. It allows air flow, but also prevents large objects from entering.
Any conductor that connects to the earth (earth ground). The term also may refer to a cabinet or chassis ground that serves as a common conductor.
ground loop
The current that flows between two or more ground connections. This is a very prevalent condition when one or more of the connections has a higher resistance than the others. This causes a difference of potential.
ground wire
Any wire that is connected to the chassis. Commonly colored green, it is usually the bare wire in electrical conduit.


hard copy
Any printout on a permanent material, such as paper, of electronically stored information.
The machinery used in a computer system e.g., disk drives, power supplies, printers, modems, etc. See also software.
A system that uses fixed wires from a particular remote device to the central processing equipment.
A frequency that is a whole-number multiple of a smaller fundamental frequency.
head, dynamic or total
In flowing fluid, the sum of the static and velocity heads at the point of measurement.
head, static
The static pressure of fluid expressed in terms of the height of a column of the fluid, or of some manometric fluid, which it would support.
1) Lines at the top of the screen that include: information about the current cell being pointed to by the cursor, a prompt line listing the next valid keystroke for various commands, and an error message line. 2) A center pipe used as a location for two or more lines to join and distribute refrigerant to various points of the system.
A form of energy that acts on a substance to cause a rise in its temperature.
heat, latent
The change of enthalpy during a change of state. With pure substances, latent heat is absorbed or rejected at constant pressure.
heat, sensible
The heat associated with a change in temperature in contrast to a heat interchange in which a change of state (latent heat) occurs.
heat, specific
The ratio of the quantity of heat required to raise the temperature of a given mass of any substance one degree to the quantity required to raise the temperature of an equal mass of a standard substance one degree.
heat content
Refer to enthalpy.
heat exchanger
A device specifically designed to transfer heat between two physically separated fluids.
heat gain, latent
Heat gain that takes place during a change of state (freezing, melting, etc.), and is not accompanied by a change in temperature.
heat gain, sensible
Heat gain that is caused by convection, conduction, or radiation, and is evidenced by an increase in temperature.
heat gain, total
The sum of all of the heat gains that is, sensible heat plus latent heat.
heat pump
A refrigerating system employed to transfer heat into or out of a space or substance. Heat pumps use direct expansion vapor compression circuits and are referred to as reverse-cycle air conditioners.
heat sink
A device used to draw off (conduct away) heat. Usually pertains to a semiconductor device. A source of reasonably constant warmth used in the heating cycle evaporator in a heat pump.
heating system, high-pressure steam
A steam heating system employing steam at pressures above 15 psig.
heating system, high-temperature water
A heating system in which water with supply temperatures higher than 2 12°F is used as a medium to convey heat from a central boiler, through a piping system, to suitable heat-distributing means.
heating system, hot water
A heating system in which water with supply temperatures of less than 2 12°F is used as a medium to convey heat from a central boiler, through a piping system, to suitable heat-distributing means.
heating system, low-pressure steam
A steam heating system employing steam at pressures between 0 and 15 psig.
heating system, panel
A heating system in which heat is transmitted by both radiation and convection from panel surfaces to both air and surrounding surfaces.
heating system, perimeter warm air
A warm air heating system of the combination panel and convection type. Warm air ducts are embedded around the perimeter in the concrete slab of a house without a basement. The ducts receive heated air from a furnace and deliver it to the heated space through registers placed in or near the floor. Air is returned to the furnace from registers near the ceiling.
heating system, radiant
A heating system in which only the heat radiated from panels is effective in providing the heating requirements. The term radiant heating is frequently used to include both panel and radiant heating.
heating system, split
A system in which the heating is accomplished by means of radiators or convectors supplemented by the mechanical circulation of air (heated or unheated) from a central point. Ventilation may be provided by the same system.
heating system, steam
A heating system in which heat is transferred from the boiler or other source of heat to the heating unit by means of steam at above or below atmospheric pressure.
heating system, vacuum
A two-pipe steam heating system equipped with the necessary accessory apparatus for permitting the operation of the system at pressures below atmospheric pressure when desired.
heating system, vapor
A steam heating system that operates under pressures at or near atmospheric, and returns the condensate to the boiler or receiver by gravity.
heating system, warm air
A warm air heating plant consisting of a heating unit (that is, an oil, gas, or electric furnace) enclosed in a casing, connected to ducts that distribute the heated air to various areas of a building and return air to the heating unit.
heating unit, electric
A structure containing one or more heating elements, electrical terminals or leads, electric insulation, and a frame or casing, all assembled together in one unit.
The unit of inductance Its symbol is H. One henry of inductance is present in a circuit when a change in current of one ampere per second induces an EMF of one volt.
Completely sealed.
hertz (Hz)
A unit of frequency equal to one cycle per second.
hexadecimal number
A number written in the base 16 number system. The first ten numbers are the integers 0 through 9, the last six numbers are the letters A through F. Also called a hex number.
high limit
The point above which the system could cause damage to the equipment or injury to persons.
high-level language
A program language that is not quite as dependent on the limitations of the computer. Some of these languages are: ALGOL, COBOL, FORTRAN, etc.
high-limit control
The control that prevents a system from reaching a dangerous level. holding current The minimum current necessary to keep an SCR or triac turned on.
horsepower (hp)
1) The unit of ability to do work, equivalent to the movement of 33,000 pounds moving through one foot in one minute. 2) A measure of the rate of doing work One horsepower is equal to 746 watts. horsepower per ton A calculation expressed as the ratio of power used to the refrigeration produced. It is found by dividing the output horsepower of the compressor motor by the tons of refrigeration produced.
hot wire
The conductor that is ungrounded, usually identified by a black or red colored insulation.
Designates any point in an electrical system that has a potential other than ground.
Refer to horsepower.
humidifier, room-spray type
An air humidifier that sprays water directly into the room.
A device used to add to and control moisture to air.
humidifier fever
Flu-like disease caused by contamination of humidifiers, vaporizers, vacuum pumps, and saunas.
To add water vapor to the atmosphere. To add water vapor moisture to any material.
A regulatory device, actuated by changes in humidity, used for the automatic control of relative humidity.
A measure of moisture in the air. humidity control A device that measures and controls the amount of moisture in the air.
humidity controller
A device that senses and controls the moisture content of air.
humidity, relative
Approximately, the ratio of the partial pressure or density of the water vapor in the air to the saturation pressure or density, respectively, of water vapor at the same temperature. More simply, the amount of water vapor actually present in the air, expressed as a percentage, compared to the maximum amount of water vapor that the air could hold under the same conditions.
Cycling above and below the set point. This behavior occurs when a controller cannot maintain the controlled variable at the desired preset level.
Heating, ventilating, and air conditioning.
An instrument used for measuring the specific gravity of a fluid.
The science dealing with the control and use of water as a heat transfer medium in air conditioning systems.
An instrument that responds to humidity conditions (usually relative humidity) of the atmosphere.
The lagging of a magnetic field behind the magnetic force that causes it. Hysteresls is caused by the friction of the atoms trying to align themselves.
Refer to hertz.


Indoor air quality. This is the measurement of acceptable air quality indoors.
Refer to integrated circuit.
I/O (input/output)
A term used to describe communication with a computer, and the data and physical components involved in that communication.
I/O, parallel
The simultaneous transfer of all the bits in a byte of information over parallel conductors.
I/O, serial
The transfer of data one bit at a time over a single wire.
immersion transmitter
A sensor equipped with an extended element, which can be inserted into a well in order to sense the temperature in liquid lines.
The total opposition to an alternating current in an electric circuit. It is the joint effect of resistance, inductive reactance, and capacitive reactance. The symbol for impedance is Z.
impedance relay
A relay that is a single-pole, single-throw normally closed relay that has a high-impedance coil and is used to lock out a circuit.
in phase
Applied to the condition that exists when two sine waves of the same frequency pass through their maximum and minimum values of like polarity at the same time.
Inch of mercury
A unit of pressure equal to the pressure exerted by a column of mercury one inch high at a temperature of 32°F.
inch of water
A unit of pressure equal to the pressure exerted by a column of water one inch high at a temperature of 39.2°F.
Any device used to indicate a condition e.g., a thermometer, pressure gauge, etc.
The property of a circuit that opposes any change in the current of that circuit.
1) The ability to produce a electromotive force by the motion of a magnetic field across a conductor. 2) The phenomenon by which room air is entrained by higher pressure supply air.
induction motor
A simple type of ac motor in which the rotor is energized by induction from the stator.
induction unit, room air
A factory-made assembly consisting of a cooling coil, or cooling and heating coil, and the means for delivering pre conditioned air.
inductive load
Any load that exhibits inductive reactance, such as a coil, motor, or transformer.
inductive reactance
The opposition to the flow of current in an ac circuit by the insertion of an inductance. It is measured in ohms.
Any coil.
Leakage of air into a building.
1) A form of radiant energy not visible to the eye. Its frequency range is less than that of visible light, and its wavelengths are longer. With specialized equipment, it can be used for transmitting wireless signals. 2) A type of radiant heat generated by heating high emissivity materials to high temperatures.
To start or open, to initiate the start-up procedures in a computer.
input module
A transducer that converts the medium variable (temperature, pressure, humidity, power) into a digital format for a microprocessor-based controller.
Any data or instructions sent to a computer or controller for processing. Also, any signal sent to a controller that will change its output.
A command that tells a computer what to do.
Material that impedes heat transfer. insulation (electrical) The non-conductive material that covers electrical wires.
Any material that will not allow electrons to pass freely through it. integer A whole number.
integral horsepower
Refers to motors rated at one horsepower or more.
integrated circuit (IC)
A complete electronic circuit manufactured as a single unit . Also called a chip.
integrated part load value
A term that pertains to the flow of water in a chiller system.
intelligent device
Any device that contains a microprocessor.
A system that links, in an organized manner, such entities as computer hardware and software.
interlock switch
A device that prevents a machine from being operated while the cabinet door is open.
An atom with less than or an excess of normal electrons.
Refer to integrated part load value. lR drop The voltage drop across a load, either resistive or inductive.
A process in which there is no change in temperature.


In motor starter terminology, to force a motor into motion by delivering power in small increments (pulses). In load shedding terminology, to prevent the power level from staying inside the deadband for an extended period of time.
A unit of energy or work. One joule is equal to one ampere flowing through a resistance of one ohm for one second.


Generally lower-case, the abbreviation for kilo (1,000).
Refer to kilobyte.
keyboard (key pad)
A device that sends encoded signals to a computer by means of depressed one or more keys.
Refer to kilohertz.
Prefix meaning one thousand (1,000). kilobyte (kB) One thousand bytes (or eight thousand bits) of information.
kilohertz (kHz)
One thousand cycles per second. kllohm (ku) One thousand ohms. kilovar (kVAR) One thousand volt-amperes reactive. kilovolt (kV) One thousand volts.
kilowatt (kW)
One thousand watts (equal to 1.34 horsepower).
Klrchhoff’s Laws
A series of laws that pertain to current and voltage in parallel circuits. More specifically: (1) The algebraic sum of the current flowing toward any point in a circuit and the current flowing away from it is equal to zero. (2) The algebraic sum of the products of current and resistance in any closed path in a circuit is equal to the algebraic sum of the electromotive forces in the path.
Refer to kilovolt.


A title given to a column or row, describing the meanings of the values in that column or row.
In a control system, the delay in response to a change in the variable being controlled, or a condition in which the sensing element of a control reacts too slowly.
core An iron core of a transformer that is made up of many flat pieces of iron or steel.
Any device that produces visible light when a current is passed through it.
large-scale integratIon (LSl)
Generally considered to have more than one hundred elements in a single integrated circuit or chip.
A mechanical device that produces intense, coherent light (light amplification by stimulated electromagnetic radiation).
latent heat
The amount of heat required to change the state of a substance e.g., changing water to ice requires the removal of 144 Btu per pound of water at sea level. Sometimes referred to as hidden heat.
latent heat of fusion
The heat required to change a solid to a liquid or vice versa at the same temperature, expressed in Btu per pound.
latent heat of vaporization
The heat required to change a saturated liquid to a saturated vapor or vice versa, expressed in Btu per pound.
leakage current
A small amount of unwanted current that travels through insulation whenever there is voltage present.
Refer to light-emitting diode.
Characters displayed with the leftmost character in the leftmost space of a cell.
Legionnaires’ disease
Legionnella. Sometimes fatal disease caused by a bacteria that can grow in parts of the HVAC system.
light-emitting diode (LED)
A semiconductor device that emits light when a current is passed through it.
limit control
A controller that keeps pressure, temperature, or humidity within certain limits, usually to protect the equipment or to prevent the development of unsafe conditions. This control is normally set above the normal operating range.
line of force
A line in a magnetic or electric field that shows its direction.
line printer
A printer that prints a complete line of information at a very high speed.
line voltage
The normal supply voltage for equipment (as distinguished from control voltage). In the U.S. and Canada, line voltages can be 120 V, 208 V, 230 V, or 460 V, either single-phase or three-phase.
linear sequence
In load shedding, first off, last on. linear valve A flow characteristic through a valve in which the seat-plug configuration is such that each equal increment of stroke will produce a change in flow proportional to the stroke movement.
Any output that varies in direct proportion to the input.
liquid fill
Refers to a temperature-sensing element that is completely filled with liquid. Any change in temperature causes the liquid to expand and contract accordingly.
liquid solenoid valve
An electrically operated valve that shuts off the refrigerant liquid line. See also pump down.
1) Any device that converts electrical energy into heat or electrical energy into mechanical energy; 2) An HVAC/R component that can be turned on and off to conserve energy. 3) In computer terminology; the ability to place a word into a register, or to place a program into the memory; 4) Any device that consumes electric power. 5) The quantity of heat to be added or removed by an HVAC/R system.
locked-rotor amperes (LRA)
The excessive current that a motor draws during starting (starting amperes) or when its rotor is locked (cannot rotate).
The recording of data.
A system of mathematical computation used in digital circuit design to produce signal responses in a predetermined, orderly fashion. It applies to truth tables, switching, and gating. The logic symbols are: and, or, not, nand, and nor.
In electronics, a complete electric circuit. In computer terminology, a series of instructions that repeats itself until a specific condition exists to terminate its operation.
low voltage
According to the National Electrical Code (NEC), any voltage below 50 V. Normally, 24 V is considered to be low voltage.
A control strategy used by a controller to prevent the variable that is being sensed from decreasing below a dangerous or undesirable condition.
low-limit control
A device that prevents a variable in a control system from going below a specified limit.
low-pressure control
Any device that opens a circuit when the pressure falls below a given point.
Refer to locked-rotor amperes.
A unit of measure relating to the amount or intensity of light produced by a light source. One lumen is equal to the flux (illumination) produced by a uniform point source of one standard candle upon each square meter of the inside surface of a sphere that has a radius of one meter. Two of these candles, if identical, would produce twice the illumination, or two lumens per square meter.


Refer to milliampere.
machIne language
A series of binary codes that instructs a computer to perform a specific function.
Any object or material that attracts iron and common steel. Refer to electromagnet.
magnetic circuit
The complete path of a magnet’s lines of force.
magnetic disk
A magnetically coated disk on which data are stored electro magnetically.
magnetic field
The space in which the magnetic force exists.
magnetic flux
Similar to lines of force, imaginary lines used for convenience to designate the direction in which magnetic forces are acting.
magnetic poles
Those sections of a magnet (called the north pole and the south pole) where the flux lines are concentrated and toward which they converge.
magnetic tape
Plastic tape that has a magnetic coating on which data can be stored.
main line
A large pipe that distributes fluids or liquids to smaller branch pipes.
1) The tubing in a pneumatic control system that carries the supply pressure from the air supply equipment to the controllers. 2) Primary pipes (hydronic) or ducts (air side) between prime movers and branch distribution.
manipulated variable
A condition or quantity controlled or regulated by an automatic control system.
An instrument used for measuring pressure, usually a U tube filled with a liquid. The liquid may be water, oil, or mercury;
manual reset
Any control that has to be physically reset after it locks out.
manual switch
A switch that is manually turned on or off, example a light switch.
The quantity of matter in a body, measured by its resistance to acceleration and proportional to its weight.
master controller
A controller that senses the condition of a variable and sends its signal to a second controller, thereby changing the set point of the second controller. See also sub master controller.
measured variable
Any condition measured by the sensing element (temperature, humidity, pressure, etc.).
mechanical efficiency
The percentage obtained by dividing the output energy of a machine by its input energy; both expressed in the same units of energy.
mechanical equivalent of heat
An energy conversion ratio of 778 Btu = 1 ft. lb.
medium-scale integration (MS1)
A micro-circuit that contains more than 12 and fewer than 100 elements.
meg (or mega)
Prefix meaning one million (1,000,000). When measuring computer memory 1,048,576 units or bytes of information. Abbreviated M.
megabyte (MB)
One million (1,000,000) bytes of information.
megahertz (MHZ)
One million (1,000,000) cycles per second.
Refer to meg ohmmeter. megohm (Me) One million (1,000,000) ohms.
meg ohmmeter
An instrument that measures very high insulation resistance. See also megger.
The circuit or system that stores information, either in a computer or in a peripheral device such as a printer or disk.
memory, bubble
Magnetic materials called garnets are what make up bubble memory. By applying a magnetic field, very small (2 to 5 microns) magnetized regions are formed. When viewed through polarized light under a microscope, they look like bubbles. Bubble access time is rather slow compared to semiconductor memory, but offers the advantages of non-volatility; low power dissipation, and no moving parts.
memory, dedicated
A special reserved location in the main memory e.g., real-time programs and program interrupts.
memory, dynamic
A special semiconductor memory that must be refreshed or recharged on a periodic basis to inhibit the loss of data.
memory, main
The memory that the central processing unit accesses directly. Also called the core or main storage.
memory, non-volatile
A memory system that will not lose its information if power is lost to the system.
memory, off-line
A memory that is not under the control of the CPU.
memory, on-line
A memory that is under the control of the CPU.
memory, programmable
Any memory that can be written to or read from a processor. Synonymous with RAM.
memory, volatile
Any form of computer memory that loses its stored information when power is lost.
memory capacity
The maximum amount of information that a storage device can retain.
mercury bulb
A type of switch that uses a small quantity of mercury in a sealed glass tube to make or break an electric circuit.
metal-oxide semiconductor
Refer to MOS. mho 1) The reciprocal of resistance. See also conductance. 2) The unit of conductance (the reciprocal of the ohm).
Prefix meaning one one-millionth (0.000001). mlcroammeter A meter that is sensitive enough to measure currents in the microampere range.
mlcroampere (pA)
One one-millionth (0.000001) of an ampere.
A unit of pressure equal to one one- millionth (0.00000 1) of the pressure of the atmosphere.
A computer designed around a single chip or integrated circuit. It usually contains some ROM, some RAM, and some I/O interfaces. Most have 8- to 16-bit word lengths.
microfarad (pF)
One one-millionth (0.00000 1) of a farad (a common rating for capacitors).
A unit of measurement equal to one one- millionth (0.000001) of a meter, or one one- thousandth (0.00 1) of a millimeter, or three hundred-ninety four hundred thousandths (0.00009) of an inch.
A miniaturized computer CPU located on a single chip.
One one-millionth (0.00000 1) of a second.
microvolt (pV)
One one-millionth (0.00000 1) of a volt.
One one-thousandth of an inch (0.001 in.).
Prefix meaning one one-thousandth (0.00 1). mllllammeter An instrument that can measure currents in the milliampere range.
mililampere (mA)
One one-thousandth (0.001) of an ampere.
One one-thousandth (0.00 1) of a second.
millivolt (mV)
One one-thousandth (0.00 1) of a volt. milliwatt (mW) One one-thousandth (0.001) of a watt.
mixed air
Air that is composed of outside air and return air.
mixing box
A device used to mix outside and return air.
mixing valve
A three-way valve (with two inlets and one outlet) that can direct full flow from either of the two inlets or proportion the flow from the two inlets.
A set of interrelated formulas and data that you create and use to assess the results of changes or possible changes in variables.
A device that connects a computer to telephone lines. It is a contraction of the words modulator-demodulator.
The form of adjustment by increments and decrements.
modulatIng controllers
Devices that are constantly repositioning themselves in proportion to the system requirements.
The act of changing the shape of a carrier sine wave by combining it with a signal of changing frequency, phase, or amplitude.
moisture content
The amount of moisture or water vapor in a given amount of air, expressed in grains.
The smallest physical unit of an element or compound.
Abbreviation for metal-oxide semiconductors, a form of transistor technology in which low costs and high densities are achieved.
A device that creates a rotating motion through the use of electricity or another medium.
motorized damper
A damper that has an actuator linked to it in such a manner as to make it normally open or normally closed.
motorized valve
A valve body that has an actuator mounted to it. The selection of the valve body determines whether the valve is normally open or normally closed.
Refer to medium-scale integration. MTBF Mean time between failures, the average length of time between equipment or device failures.
Mean time to repair, the average time required to diagnose and also to repair faulty equipment.
A single meter that combines the functions of an ammeter, a voltmeter, and an ohmmeter.
mush coil
A coil made with round wire. mutual Inductance 1) A circuit property that exists when the relative position of two current-carrying conductors causes the magnetic lines of force from one to link with those of the other. 2) The induction of the lines of force from one conductor to another conductor.
Refer to millivolt.
Refer to milliwatt.


Prefix meaning one one-billionth (0.000000001). nanosecond One one-billionth of a second.
National Electrical Code.
The terminal of a battery or power supply that has an excess of electrons. Is non-positive and below zero. See also positive.
negative charge
The electrical charge of a body that has an excess of electrons.
negative ion
An atomic particle that has a negative charge because it contains more electrons than protons.
negative temperature coefficient
Pertains to the relationship of resistance to temperature in a thermistor i.e., as the temperature increases, the resistance decreases.
negative terminal
The terminal of a battery or power supply that has an excess of electrons, and thus is said to be negatively charged.
National Electrical Manufacturers Association.
net refrigerating effect
The net refrigerating ability of a refrigerant, in Btu per pound, found by subtracting the enthalpy of the liquid entering the expansion device from the total heat of the vapor as it leaves the evaporator.
neutral bus bar
The bus bar in an electrical panel to which all the bare wires are connected. The neutral bus is at ground potential.
neutral wire
The conductor (commonly white wire) that is grounded at the distribution panel.
A particle in the nucleus of an atom that has no electrical charge, but a mass equal to that of a proton plus an electron.
In computer terminology; a group of four bits, or one half of a byte.
night setback
The adjustment of a temperature control during unoccupied hours to reduce operating costs.
National Institute for Occupational Safety and Health.
N-channel MOS, a MOS circuit in which electrons are the charge carriers. This system is faster than PMOS.
Electrical or electronic interference with a signal. Any undesired sound or frequency encountered in a signal system.
nonvolatile RAM (NVRAM)
A random access memory that does not lose its information when its power supply is turned off.
nonvolatile storage
A storage system that retains its information even when there is a loss of power. Contrast with volatile storage.
normally closed (N/C)
A switch, relay, valve, or damper that is closed when the power is not being applied.
normally open (N/O)
A switch, relay, valve, or damper that is open when the power is not being applied.
The center segment of an atom, consisting of protons and neutrons. null The zero level in an electric circuit.
Refer to nonvolatile RAM.


Refer to open-circuit voltage.
Abbreviation for open drip-proof, which is a type of motor that has an open frame, but drops of liquid falling at a 15° angle will not affect its operation.
off-line 1)
Any piece of equipment that is effectively turned off, either by the CPU or by human means. 2) Not connected to a network.
off-cycle defrost control
A control whose cut-out setting can be adjusted, but whose cut-in remains the same.
A term used to describe the difference between the control point (sometimes called the operating point) and the set point. Note: In a proportional control system, offset is a result of the load being greater or less than 50% of capacity
The unit of electrical resistance. One ohm is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere. Its symbol is L Ohm’s Law A series of laws pertaining to the relationships of current, voltage, and resistance. ohmmeter An instrument that measures electrical resistance.
ON/OFF control
Any two-position controller that allows operation only in one of the two extreme positions.
1) Any piece of equipment that is effectively turned on, either by the CPU or by human means. 2) Connected to a network.
open circuit
Any electric circuit that does not have a complete path for the electrons to flow.
open-circuit voltage (OCV)
The voltage read at the output of a transformer or power supply without a load.
open drip-proof
Refer to ODP.
open loop
A system in which there is no self correcting action for errors, as there is in a closed loop system.
operating point
The value of the controlled condition at which the controller actually operates.
operating system
A set of programs that manages the overall operation of a computer. operator A mathematical or logical symbol, an operation to be performed, or a relationship to be tested.
optical isolation
1) A method of protecting sensitive electronic circuits by using light emitters and receptors for the purpose of connecting with input and/or output devices. 2) A system in which sensitive electronic circuits are protected by light emitters and receptors.
An opening in a tube designed to regulate the flow of a fluid.
The process of giving off fumes. output module Any device that is used to interface the microprocessor-based controller with the user’s devices. The module contains circuitry that will convert the digital output signals to voltages, currents, or on/off levels compatible with the user’s devices.
In computer terminoIog processed information that is delivered by a computer. In electronics, the signal or power delivered by a system.
overcurrent device
A fuse or circuit breaker type of device that opens a circuit when the devices current capabilities are exceeded.
A condition that occurs when the full-load allowable current level of a device is exceeded.
The amount of deviation from the set point that occurs before a controlled variable stabilizes, after a change of input. When a control system has a tendency to fluctuate above and below set points, the system requires longer response times to adjust for changes.


parallel circuit
A circuit in which all the devices have the same voltage impressed on them.
A control arrangement in which several actuators, all with the same spring ranges, move through their strokes in unison as the signal from the controller changes.
The specific characteristics of a system or device that describe its operational limits or the acceptable range of values for best performance.
parity check
An error detection system used in computers.
part wind start (PWS)
A type of motor that has two separate starting devices, such as contactors or starters. When starting is required, one contactor energizes the first winding. Then, after a specific period of time (usually one to two seconds), a second contactor energizes the second winding. partial pressure The absolute pressure of one of the component gases in a mixture of gases. particulates Particles of solids Or liquids in the air.
1) A high-level computer language, named for Blaise Pascal (1623—1662), a French mathematician. 2) The unit of force per unit area.
1) Polychiorinated biphenyl, an industrial chemical compound once used extensively as a dielectric in run capacitors and transformers. It was found to cause cancer and is no longer used. 2) Printed circuit board, a board made of plastic or similar material on which flat copper or aluminum foils are used in place of wires.
peak power
The maximum value of power output. peak-to-peak voltage The maximum voltage or current values represented by a sine wave, equal to 1.414 times the RMS level (Epe = EeffX 1.414).
Refer to pneumatic-electric switch. percent authority The adjustment of a receiver controller that determines the effect of the reset signal of the secondary transmitter as a percentage of the signal of the primary transmitter.
performance factor
The ratio of the useful output capacity of a system to the input required to obtain it. Units of capacity and input need not be consistent.
Any device that is connected to a CPU, such as a monitor or printer. permalloy An alloy of nickel and iron that has very high magnetic properties.
permanent magnet
A piece of material that is magnetized to such an extent that it retains its magnetism indefinitely.
permanent split-capacitor (PSC)
A type of electric motor that, by the use of one capacitor in the circuit at all times, achieves good running efficiency and moderate starting torque.
The measure of the ability of a material to act as a path for magnetic lines of force, compared to that of air. (Air has a permeability value of 1.)
Refer to picofarad.
The relationship, expressed as an angle, between two alternating currents or voltages when current or voltage is plotted as a function of time.
phase angle
The number of degrees by which one wave either leads or lags another. Also called phase difference.
phase shift
The difference between corresponding points on input and output signal waveshapes, expressed in degrees.
A semiconductor device that will either allow or restrict the flow of current in response to the application of light.
A packet of light energy used to describe the particle-like characteristics of light.
pickup current
The amount of current needed to cause a relay to energize.
pickup voltage
The amount of voltage needed to cause a relay’s coil to energize and pull in its armature or solenoid.
(p9 One micro-microfarad (a millionth of a millionth of a farad).
Refer to control, proportional/integral/derivative (PID).
The characteristic of some minerals, primarily quartz crystals, to exhibit current generated when pressure is applied. Conversely, they will vibrate when a voltage is applied to them.
pilot duty
Refers to a relay in which a low voltage (e.g., 24 V) at a low current (usually around 1 or 2 A) controls a larger voltage and current.
pipe, copper or brass
Seamless tube conforming to the particular dimensions commercially known as standard pipe sizes.
plenum chamber
An air compartment connected to one or more distributing ducts.
P-channel MOS, a MOS circuit in which holes are the charge carriers. This is the oldest type of MOS circuitry.
Operated by air pressure. pneumatic-electric switch (PE) A pneumatically operated switch in which the contacts are made or broken in order to operate electrical devices in a pneumatic control system.
The property or characteristic of having two opposite magnetic poles, one north and the other south. The term also applies to the directions (north or south) of an electric field.
1) An electrode of a battery, or the point at which the lines of force are concentrated on a magnet. 2) The number of separate circuits on a switch or relay.
pole pieces
The shaped magnetic material upon which a motor’s stator windings are mounted or wound.
A signal sent from a central processing unit to tell a remote device to transmit data.
A routine that invites another device or remote station to transmit data. Contrast with selecting.
polychlorinated biphenyl
Refer to PCB. polyphase A circuit that uses more than one phase, usually three-phase, of alternating current. Also called multi phase.
polytropic change
Any set of changes in a gas. Most commonly represented by the equation PV’ = constant.
Pontiac fever
Mild form of Legionnaires’ disease caused by a different bacteria.
The opening in the seat of a valve. Also, the I/O connection on a computer.
A body that is deficient in electrons. positive charge The electrical charge of a body that has a deficiency of electrons.
Ion An atomic particle that has a positive charge because it has lost one or more electrons and thus contains an excess of protons.<.dd>
positive positioning relay
An auxiliary device that accepts a signal from a controller and, in response, sends a signal on to the actuator. This action will increase the pressure to the actuator at a magnitude (up to main pressure) that will extend the actuator to the point called for by the controller signal or, conversely, decrease the pressure to the actuator at a magnitude (down to zero pressure) that will retract the actuator to the point called for by the controller signal.
positive positioning
A characteristic of a controlled device in which the device has the maximum force available at any point of its stroke.
positive temperature coefficient
Pertains to the relationship of resistance to temperature in a thermistor i.e., as the temperature increases, the resistance increases.
relay A switch that is normally closed - operates on the counter electromotive force (CEMF) of a motor to drop out and take the start capacitor out of the circuit.
The difference in electrical energy between one body and another body, measured in volts. The difference in voltage between two points of a circuit.
1) An instrument used for comparing small electromotive forces, or for measuring small electromotive forces by comparison with a known electromotive force. Its principal advantage is that, during the measurement, no current flows through the source of electromotive force. 2) Such an instrument used to position a modulating motor.
pounds per square inch (psi)
A unit of measurement for pressure. Often used to express atmospheric pressure, or the pressure exerted by a liquid or a confined gas on a given area. pounds per square inch, absolute (psia) Refer to absolute pressure.
pounds per square inch, gauge (psig)
Refer to gauge pressure.
The rate of performing work (amount of work per unit of time). Common units are horsepower, Btu per hour, and watts.
power consumption
Power used multiplied by time, often measured in kWh.
power factor
The ratio of true or actual power as measured by a wattmeter to the apparent power as measured by a voltmeter and an ammeter in an alternating current circuit.
power fall/restart
A system that allows a computer to restart after a power failure.
power stroke
The extension of the actuator shaft as a result of the signal from the controller exceeding the opposing force of the spring.
An indication of how accurate a value can be measured by an instrument.
In air conditioning, to heat the air in advance of other processes.
The normal (perpendicular) force exerted by a gas or liquid on the walls of its container, normally measured in pounds per square inch (psi).
pressure, absolute
The pressure referred to that above absolute zero. It is the sum of the gauge pressure and the atmospheric pressure.
pressure, atmospheric
The pressure exerted by the atmosphere, as indicated by a barometer. Standard pressure at sea level is 14.696 psi or 29.921 inches of mercury at 32°F.
pressure, back
The pressure in the low side of a refrigeration system. Also called suction pressure or low-side pressure.
pressure, gauge
Pressure above atmospheric pressure, measured with atmospheric pressure as a base.
pressure, total
In the theory of the flow of fluids, the sum of the static pressure and velocity pressure at the point of measurement. Also called dynamic pressure.
pressure, velocity
In moving fluid, the pressure capable of causing an equivalent velocity; if applied to move the same fluid through an orifice such that all pressure energy expended is converted into kinetic energy.
pressure control
Any device that opens or closes a circuit, either electrically or pneumatically. At a pre-determined set point or that point selected by the technician.
pressure drop
The difference in pressure across a restriction, intentional or otherwise. pressure-limiting device A pressure-responsive mechanism designed to stop automatically the operation of the pressure-imposing element at the predetermined pressure.
pressure relief device
A valve or rupture member which is designed to relieve excessive pressure automatically.
primary control
A device that directly or indirectly controls the control agent in response to the needs of the controller. Usually a motor, a valve, or a relay.
primary element
The sensor part of a controller e.g., the bimetal element in a thermostat that senses a change in the controlled variable and initiates the necessary corrective action.
Any device that prints characters on a hard copy medium, such as paper.
progressive sequence
In load shedding, it is first off, first on.
Programmable read only memory a type of memory that can be programmed by the user. However, it can only be programmed once.
prompt line
The line in the header indicating what to do next.
proportional action
A type of control in which the output of a device is moved or controlled in direct proportion to the input.
proportional band
The fraction of the controller span required to cause an actuator to be driven through its complete stroke.
proportional control
A control system that corrects the output by adding feedback to the input. It is always proportional to any variations in the input of the controller.
A positively charged particle in the nucleus of an atom.
Refer to permanent split-capacitor. psi Refer to pounds per square inch. psia Refer to absolute pressure.
Refer to gauge pressure.
A device for measuring the humidity or hygrometric state of the atmosphere. Normally the instruments have an indexed scale to allow direct conversion from the temperature readings to the percentage of relative humidity.
A flat wheel attached to a motor with either a adjustable or fixed V-shaped groove.
pull-out torque
The maximum torque that a motor can deliver without stalling. Also called breakdown torque.
pump down
When either the king valve is closed or the liquid line solenoid is de-energized allowing the compressor to pump most of the refrigerant into the receiver and shuts off on a low-pressure control. Also called pump out.
pushbutton switch
A switch that is operated manually by pressing on it.
Refer to part wind start.
An instrument for measuring extremely high temperatures.


A flow characteristic through a valve whereby the maximum flow is approached rapidly as the valve begins to open.


An enclosure, channel, or conduit for holding conductors, cables, or optical fiber bundles.
Heat flow without contact between the hot and cold materials. Also, heat flow by electron flow, even across a vacuum.
The base of a number system. For example, in the decimal system the radix is 10, because there are 10 symbols used (0, 1, 2, 3, 4, 5, 6, 7, 8, 9).
Radioactive gas produced by the decay of radium in natural materials.
Refer to random access memory.
A current or voltage that varies at a uniform or constant rate.
random access memory (RAM)
A temporary memory for storing data in a computer.
The span of values from one point of reference to another e.g., a thermostat’s range may be from 50 to 90°F The difference between the minimum and maximum points of operation of a properly functioning control.
The ratio of maximum to minimum flow of a fluid.
rated-load amperes (RLA)
The maximum current that a motor can draw under any operating condition. Rated load current is a function of the motor and the motor protector.
The comparison of one set of values to another set of values. Mathematically, the relation between two quantities expressed as the quotient of one divided by the other.
The opposition afforded an alternating current circuit, measured in ohms.
To move a worksheet, life, or data from disk storage to your display screen.
read-only memory (ROM)
A type of permanent memory used in computers that allows commands to be stored and accessed at any time. It is a memory that can only be read from nothing can be written to it.
real time
Refers to the actual time in which an event actually happens.
receiver controller
A device that receives small signal changes from a transmitter and amplifies these small changes to an output of 3 to 13 psi to the controlled device.
To reprocess refrigerant to new product specifications by means that include distillation. Reclaiming requires chemical analysis of the refrigerant to determine that the appropriate specifications have been met. This term usually implies the use of processes or procedures available only at a reprocessing or manufacturing facility.
To remove refrigerant in any condition from a system and store it in an external container (without necessarily testing or processing it in anyway).
A device for converting alternating current to direct current. See also diode.
To clean refrigerant for reuse by means of oil separation and single or multiple passes through devices such as replaceable core filter-driers that reduce moisture, acidity; and particulate matter. This term usually applies to procedures implemented at the field job site or at the local service shop.
regenerative heating (or cooling)
This is the process of utilizing heat that must be rejected or absorbed in one part of the cycle to perform a useful function in another part of the cycle, by heat transfer.
1) A short-term storage circuit in a computer where a certain amount of data can be stored. 2) A grill or diffuser covering.
The process of adding heat to already cooled air to maintain humidity levels.
relative humidity (RH)
The ratio, expressed as a percentage, of the amount of moisture that is present in the air to the amount that can exist in the air at that temperature and pressure.
An electrical switch that uses a small amount of current or voltage to control a larger amount of current or voltage.
relay contact bounce (chatter)
The unwanted rapid opening and closing of a set of relay contacts, usually caused by a low-voltage condition on the relay’s coil.
relay control
A pneumatic control arrangement in which the main air to the controller passes through a valve mechanism to the controlled device. The valve mechanism is operated by the pressure variation of the pilot chamber.
A measure of the opposition that a material offers to magnetic lines of force. remote-bulb thermostat A thermostat in which the sensing element is filled or partially filled with a fluid and connected to a bellows by a capillary. The expansion and contraction of the fluid acts on the bellows to operate the switch.
Copy information in a cell or cells into another cell or cells.
To change the set point of a controller automatically by the use of a second sensor. Not to be confused with automatic reset. Also, to close an opened circuit breaker.
reset control
1) Changing a controller’s set point by means of a signal from a second controller or transmitter. This is the result of a change in the variable that the second controller is measuring. 2) Two controllers operating together, one sensing a condition other than the controlled space, and changing the set point of the second controller.
reset ratio
The change in outdoor temperature to the change in the indoor temperature control point. For example, if the reset ratio is 2:1, then for every 2°F outdoor change, the indoor temperature would change 1°F.
A program located in a computer’s memory (usually in the main memory). A person who inhabits a conditioned space.
The opposition to the flow of an electric current caused by certain physical characteristics of a conductor, measured in ohms.
resistance, thermal
The reciprocal of thermal conductance or thermal transmittance.
resistance temperature detector (RTD)
A resistor, usually made of platinum wire, that is affected by changes in temperature. Used for measuring temperature with a special resistance thermometer. resistive load A load that contains only resistance. (It does not contain any capacitive or inductive reactance.)
resistive rating
The rating of a control that does not use an inductive load.
A component that acts to oppose the flow of current in an electric circuit.
response time
The time required for a computer to respond to an input from one of its peripherals.
A device with a very small opening that changes the velocity pressure of the line return, dry A return pipe in a steam heating system that carries both water of condensation and air. The dry return is above the level of the water line in the boiler in a gravity system.
return, wet
That part of a return main of a steam heating system that is filled with water of condensation. The wet return usually is below the level of the water line in the boiler.
return mains
Pipes or conduits that return a heating or cooling medium from the heat transfer unit to the source of heat or refrigeration.
return stroke
The retraction of the actuator shaft as a result of a signal from the controller decreasing below the opposing force of the spring.
The persistence of sound in an enclosed space after the source has stopped.
reverse-acting (RA)
An instrument in which the output signal changes in the opposite direction of that in which the controlled variable changes i.e., the response decreases as the change in variable increases.
reversing relay
A relay that will operate in a manner opposite that of its normal operation.
Refer to relative humidity.
A type of variable resistor used for adjusting the current in a circuit.
Characters displayed with the right most character in the rightmost space of a cell.
Refer to rated-load amperes.
RMS value
The root-mean-square value of an alternating current voltage or power sine wave, calculated as the square root of the average of the squares of all the instantaneous values throughout one complete cycle.
Refer to read-only memory.
Rotation, either clockwise (CW) or counter clockwise (CCW), as listed per NEMA, and viewed from the shaft end.
The revolving part of a motor. routine A set of special coded instructions that tells a computer what to do and when to do it.
Revolutions per minute.
RS-232, RS-422, RS-484
A series of configurations developed by the EIA, each of which establishes an interface of connections between a CPU and its peripherals.
Refer to resistance temperature detector.
run time
The amount of time required for a computer to run a program. Also, the amount of time required for a refrigeration or air conditioning system to maintain the load at given conditions.


Seasonal affective disorder. Health problem caused by lack of sunlight during winter months.
1) The condition of a liquid at its boiling temperature, and of a vapor at its condensing temperature. 2) The condition for the coexistence of a vapor and a liquid for example, steam over the water from which it is being generated.
Sick building syndrome. A group of symptoms that are relieved by leaving the building. scaling The conversion of a standard signal into an appropriate unit of measurement e.g., converting 4 to 20 mA to 40 to 260°F.
A wiring diagram that shows by the use of symbols and lines the functions and connections of an electric pneumatic refrigeration, or air flow circuit.
Refer to silicon-controlled rectifier.
The next new line of information following the last line on a CR1 screen. As the last line fills up the screen, the first line is. replaced.
sealed VA
The volt-amperes after a controlled device, such as a relay or a contactor, has been operated.
The stationary portion of a valve. secondary windIng The coil of wire on a transformer that receives its voltage and current by mutual inductance.
The process of inviting another device to receive data. See also polling.
selecting relay
An auxiliary device that chooses the higher or lower (depending on the specific model) signal from several controllers and then passes that signal on to the controlled device.
self-actuated control
A type of control system in which the source of energy is derived from the variable under control, rather than from an external source.
The creation of an electromotive force in a circuit by its own magnetic field.
A material that is neither a conductor nor an insulator. Silicon and germanium are the two most common semiconductor materials.
sensible heat
The heat that can be measured with a thermometer. Heat energy that changes the temperature of a substance without changing its state.
sensing device
A device that keeps track of a measured condition and its changes. It then signals a controller to make a change. A bulb on a thermostat is a sensing device.
The capability of a controller to measure and respond to variations in a controlled condition.
1) A device used to detect and measure changes in a variable, and transmit the information as an analog signal to a central processor. 2) Any device that can measure analog data and transmit the information to a central processor. This can be either proportional or two- position. sequencing A control arrangement in which several actuators move through their strokes in succession as the signal from the controller changes. This arrangement is derived by using actuators with different spring ranges, end switches, rotating switches, or by using a pneumatic relay.
series circuit
A circuit in which the components are connected one after another, and the current is the same throughout the circuit.
service factor
The reserve margin built into a motor e.g., a service factor of 1.25 means that the motor can deliver 25% more that its rated horsepower without dangerously overheating.
set point
The point at which the desired value of the controlled variable is set.
Refer to service factor.
shaded pole
1) A type of small induction motor that has no starting winding or components. It uses copper rings to phase shift the incoming current to the motor windings. 2) A single-phase induction with one or more auxiliary short- circuited stator windings displaced magnetically from the main winding.
shading ring
Copper ring set into the pole piece of an ac motor to produce the phase shift for starting.
short circuit
An unintentional current path between two components in a circuit, or between a component and ground.
sick building
A building in which a significant number of the occupants complain that their environment may be causing physical symptoms.
Electrical information that can be transmitted through wires or by electromagnetic waves.
significant digit (SD)
A digit that contributes to the accuracy of a number. SDs are counted from the first digit on the left that is not a zero to the last accurate digit on the right. (A right-hand digit may be counted if it is an accurate part of the number.) For example, the number 2,300.0 has five SDs, but 2,300 probably has only two (it is not known that the last two digits are precise). The number 2,301 has four SDs, and 0.0023 has two.
A semiconductor material that is often used to make transistors and integrated circuits. It can withstand higher temperatures than germanium.
silIcon-controlled rectifier (SCR)
A semiconductor device that functions as an electrically controlled switch by conducting current when the gate is triggered.
single phasing
The condition that occurs when one phase of a three-phase system opens.
One set of electrical contacts, that will make (open) or break (close) on a switch operation.single-pole, double-throw switch
A three-terminal switch or relay contact that either breaks or makes (opens or closes) one circuit.
single-seated valve
A valve configuration in which there is one valve plug that comes to rest on one surface to obtain close-off.
Switch or relay contacts that will make or break in only one direction.
skip day
A designated day on which a particular program schedule will not function.
The difference between the synchronous speed and the actual speed of the rotor in a motor.
small-scale integration (SS1)
Integration of less complexity than medium-scale integration (MSI).
An air suspension of carbon or soot particles less than 0.1 micron in size.
snap-action controllers
Controllers that snap open or closed when operated (usually done with a magnet or bimetal strip).
soft copy
An output presented on a CRT screen soft start An additional solid state start device added to a PSC compressor motor to increase starting torque. See also part wind start.
Any written program or routine to be used with digital equipment. See also hardware.
A coil of wire which when voltage is applied creates a magnetic coil that moves an iron core plunger.
A field of electronics that uses semiconductor materials and has no moving parts. It requires less power than conventional systems.
solid-state devices
Electronic devices that use semiconductor materials and have no moving parts. They require less power than vacuum tubes and are specifically prepared solid matter.
One sone is defined as the loudness of a 1,000-Hz signal with the pressure of 40 decibels.
source range
Single cell or group of cells from which information is duplicated to corresponding cells of the target range.
space thermostat
A thermostat whose sensor is in the controlled space.
The difference between the minimum and maximum set point capability of a controller, the difference between the minimum and maximum sensing capability of a transmitter, or the difference between the start point and finish point of an actuator. Examples: For a controller with a range of 55 to 85°F, the span is 30°F For a transmitter with a range of 40 to 240°F, the span is 200°F For an actuator with a range of 5 to 10 psi, the span is 5 psi. See also range.
span (actuator)
The change in the controller’s output needed to drive the actuator through its complete stroke.
span (controller)
The difference between the highest and lowest value of the set point adjustment.
specIfIc gravity
The ratio of the density of a substance to the density of water at a given temperature. See also hygrometer.
specIfic humidity
The weight of water vapor associated with one pound of dry air. Also called humidity ratio.
specific volume
The volume of a material in cubic feet per pound, calculated as the reciprocal of density.
Refer to transient.
split screen
A feature that allows two non contiguous sections of the worksheet to be viewed at the same time.
spilt phase
A type of motor that has two separate windings and is controlled by a switch or relay.
split-phase motor
A single-phase induction motor that has an auxiliary winding connected in parallel with the main winding.
squirrel cage
A type of rotor windings in which conductors are imbedded in the rotor body. The conductors are shorted together at the ends by continuous rings.
Refer to small-scale integration.
stack height
The height of a gravity convector between the bottom of the heating unit and the top of the outlet opening.
standard ton conditions
Refrigerant at a rate of one ton of ice melting per 24 hours (288,000 Btu per 24 hours, 12,000 Btu per hour, or 200 Btu per minute) in a system operating at 5°F evaporating, 86°F condensing, liquid sub cooled 9°F, and suction gas superheated 9°F.
start point
The pressure necessary to begin compressing the spring of an actuator, thereby causing the actuator to begin its stroke. Example: For an actuator with a spring range of 5 to 10 psi, the 5 is the start point. See also finish point.
start relay
A relay that controls the start winding of a motor through the use of applied voltage or current.
A contactor that has some form of overload protection built into it. starting torque The amount of torque produced when power is applied to a motor at rest.
static electricity
Electricity in the form of a stationary (unmoving) charge. Usually produced by friction between two surfaces in close contact with each other.
static pressure
The force per unit area caused by the bursting force of the air against the walls of a duct as air is moved through the duct. Frequently expressed in feet of water column.
static pressure rating
The maximum pressure (from inside the body to outside the body) that a valve will tolerate.
The stationary (non-rotating) part of an electric induction motor, on which the coils are wound.
steam, dry saturated
Steam at the saturation temperature corresponding to the pressure, and containing no water in suspension.
steam, superheated
Steam at a temperature higher than the saturation temperature corresponding to the pressure.
steam, wet saturated
Steam at the saturation temperature corresponding to the pressure, and containing water particles in suspension.
steam trap
A device for allowing the passage of condensate, or air and condensate, and preventing the passage of steam.
The shaft that moves the plug from or onto the valve seat.
stepper motor
A type of dc motor that provides precise control by operating in a series of discrete steps of uniform magnitude.
A general term given to any device that can retain or hold electronic information for an extended period of time.
storage capacity
The maximum amount of data that can be retained in a computer’s hard drive or other storage device.
A wire or switch used for making electrical connections. See also bus bar.
strap-on thermostat
A controller designed for mounting on and sensing the temperature of a surface, such as that of a pipe.
The distance traveled by a piston, valve stem, or crank ann.
sub-cooled liquid
Liquid refrigerant at a temperature below the saturation temperature corresponding to its pressure.
The process of a solid changing directly to a vapor without passing through a liquid state for example, dry ice.
submaster controller
A controller that receives a signal from a master controller. The signal from the master controller changes the set point of the submaster controller.
A portion of a computer program that is stored only once in the memory, but can be used repeatedly when called by the running master routine.
summer/winter thermostat
A pneumatic thermostat that can be indexed by a change in main pressure to control the same device at one action during the summer (e.g., 15 psi, reverse action) and another action during the winter (e.g., 20 psi, direct action).
summer/winter switch
Usually a fan switch on a furnace used to provide air conditioning in the summer.
That portion of the heat in a gas that raises its temperature above the saturation temperature corresponding to its pressure.
supply mains
The pipes through which the heating or cooling medium of a system flows from the source of heat or refrigeration to the runouts and risers leading to the heating or cooling units.
Any device used to open (interrupt the current) or close (complete a path for the conduction of current) an electric circuit.
switching relay
An auxiliary device that chooses either of two signals to be the output to the controlled device. The choice is made based on the signal received by the pilot chamber.
Occurring concurrently and with regular time periods (frequency). The opposite of asynchronous.
synchronous communication
Data transmitted at a fixed rate or frequency The receiver and transmitter use the same clock signals for synchronization.
synchronous speed
The speed at which the rotating field in an ac motor revolves. This speed is a function of the number of poles in the field and the frequency of the applied voltage.
The structure of a command or formula. The order in which the parts of the command or formula appear and the punctuation used.
system, direct-return
A piping arrangement for a heating system in which the heating fluid is returned to the boiler by the shortest direct path, resulting in considerable differences in the lengths of the several circuits composing the system.
system, down-feed
A piping arrangement for a heating system in which the heating fluid is circulated through supply mains that are above the levels of the heating or cooling units which they serve.
system, forced-circulation
A heating system in which the heating or cooling fluid circulation is effected by a fan or pump.
system, gravity circulation
A heating system in which the circulation is effected by the difference in densities of cooler and warmer fluids in the two sides of the system.
system, one-pipe
A piping system in which the fluid withdrawn from the supply main passes through heating or cooling units and returns to the same supply main.
system, reverse-return
A piping system in which the heating or cooling medium from several heat transfer units is returned along paths arranged so that all circuits composing the system or composing a major subdivision of it are of effectively equal length.
system, two-pipe
A piping system in which the fluid withdrawn from a supply main passes through heating or cooling units to a separate return main.
system, up-feed
A piping arrangement for a heating, air conditioning, or refrigerating system in which the cooling fluid is circulated through supply mains that are located below the levels of the heating or cooling units which they serve.


Testing and balancing (an HVAC system).
An instrument for measuring the speed of a rotating device, such as a motor.
target range
Single cell or group of cells to which information from corresponding source range cells is replicated.
task lighting
Light fixture at work level.
Refer to time-delay relay.
Totally enclosed, a type of motor used in dirty or damp locations (but not waterproof or explosion- proof).
Totally enclosed, fan-cooled. temperature, ambient Generally speaking, the temperature of the air surrounding an object.
temperature, dew point
The temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature of the vapor is reduced. The temperature corresponding to saturation (100% relative humidity) for a given absolute humidity at constant pressure.
temperature, dry-bulb
The temperature of a gas or mixture of gases indicated by an accurate thermometer after correction for radiation.
temperature, room
The temperature of any room for example, a room in which a refrigerator is being operated or tested, or a room being conditioned for the comfort of occupants. Used colloquially to mean the ordinary temperature that one is accustomed to find in a dwelling.
temperature, saturation
For a fluid, the boiling point corresponding to a given pressure. temperature, wet-bulb 1) The temperature indicated by a wet-bulb psychrometer constructed and used according to specifications. 2) The saturation temperature of a constant-enthalpy air-moisture mixture. It can be used to determine relative humidity.
temperature difference (TD)
The difference in temperature between two substances or portions of one substance.
Totally enclosed, non-ventilated. terminal, computer A peripheral device that is used to communicates with a computer e.g., a keyboard.
terminal, electrical
A connection for wires to be joined together.
terminate and stay resident (TSR)
A special type of computer program that remains in the main memory after it has been executed.
A character that marks the end of input data. The terminator for ONcaic is the CR key.
A quantity of heat equivalent to 100,000 Btu. thermal comfort range Range of temperature and humidity that provide a comfortable indoor environment.
thermal relay
A relay that is opened or closed in response to changes in temperature.
A semiconductor device that changes resistance in response to a change in temperature.
A device made of two dissimilar metals that produces a voltage when heat is applied.
The science of heat energy and its transformations to and from other forms of energy.
A series of individual thermocouples connected in series, used to generate a sufficiently high voltage to operate a gas valve without the aid of outside power. (Also called self-generating.)
A device that opens or closes a circuit in response to changes in temperature, thereby causing a signal to be sent to some type of actuator.
thermostat, reverse-acting
An instrument used for activating a control circuit upon sensing a predetermined high temperature.
thermostatic trap
A steam trap that opens in response to a drop in temperature such as when cold condensate or cold air reaches it and closes when steam reaches it. The temperature-sensitive element is usually a sealed bellows or series of diaphragm chambers containing a small quantity of volatile liquid.
three-mode control
Refer to control, proportionalfintegral/derivative (PID).
A term applied to alternating current in which each of the phases is 120 degrees apart.three-way valve
A valve that has three connections with one common to the other two.
throttling range (TR)
1) The amount of change of the variable necessary for the controller to produce an output change of 3 to 13 psig. 2) The amount of change in a variable that will cause the controlled device to go through its full range.
The horizontal or vertical axial distance that an airstream travels after leaving an air outlet before the maximum stream velocity is reduced to a specified terminal level.
time-delay relay
A relay in which there is an interval of time between the energizing or de-energizing of the coil and the opening or closing of the contacts.
The twisting or turning force that a rotating shaft produces when it is transmitting power.
total heat
The sum of all the sensible and latent heats of a refrigerant between a base temperature (usually 40°F) and the temperature of the refrigerant. Also called enthalpy.
total pressure
The sum of all the partial pressures of the gases in a mixture of gases.
totally enclosed
A term used to describe a type of motor designed to be used in dirty or damp places. The term is not used to mean waterproof or explosion-proof.
Refer to throttling range.
Any device that converts one form of energy to another form of energy.
transformation of energy
Change of energy in one form of energy into another form of energy, an example is the transformation of electric energy to mechanical energy (plus some heat energy) in an electric motor.
A device that is coupled by two or more coils of wire with the ability to transfer energy from one circuit to another.
A temporary form of electrical energy that occurs on a circuit board because of a sudden change of current or voltage. Also referred to as a spike.
A semiconductor device that can transfer a signal from one low resistance to a higher resistance, thereby producing amplification.
transmission line
Any conductor that carries energy from the source to its load.
The act of conveying energy from one point to another.
transmission, heat, coefficient
of Any one of a number of coefficients used in the calculation of heat transmission by conduction, convection, and radiation through various materials and structures.
transmittance, thermal (U factor)
The time rate of heat flow from the fluid on the warm side of a barrier to the fluid on the cold side, per unit of temperature difference.
1) A device that converts a signal (in proportion to changes in the variable being measured) to some form of energy that can be sent over wires, through fibers, through tubing, or through the air. 2) A device that measures the variable and sends a signal of 3 to 15 psig to a receiver controller and gauge, thereby accomplishing control and indication.
A solid-state switching device used to control current during both alternations of an ac cycle. Similar in construction to two SCRs back-to-back, with a common gate and common terminals.
The process of observing a system to locate and diagnose faults in equipment by means of systematic checking.
true power
The power consumed by an electric circuit, as measured directly with a wattmeter.
Refer to terminate and stay resident.
A small-diameter pipe normally made from copper, aluminum, or plastic.
tube, capillary
Refer to capillary tube.
tube, finned
A heat transfer tube or pipe with an extended surface area in the form of fins, disks, or ribs.
twisted pair
A cable composed of two small insulated conductors twisted together in a common covering.
two-position control
A method of control in which the control device is either 100% open or 100% closed, allowing the controlled medium to flow only at these respective rates. Also known as ON/OFF control.
two-stage controller
A controller that operates in two separate stages, one following the other in sequence e.g., a two-stage thermostat, in which the switches are separated by a certain temperature range, so that the first stage may be made at 68°F and the second stage at 70°F.
two-way valve
A valve that has only two connections (one inlet and one outlet) and operates only in the fully open or fully closed position, with no positions in between. This valve is sometimes referred to as a ON/OFF valve.


The invisible low frequency of the light spectrum. Used with certain chemicals for detecting leaks in refrigeration systems.
unit heater
A direct-heating, factory-made encased assembly, including a heating element, fan and motor, and directional outlet.
universal motor
A motor that can be operated either on alternating current or direct current. A universal motor normally runs at high speed and has brushes.
unload (compressor)
To disengage certain cylinders in a compressor by bypassing some of the discharge gas back into the suction section. (This action, in turn, lowers the capacity of the compressor.)
1) Closer to the power source e.g., the fuses are upstream from the disconnect box. 2) In regards to air water or refrigerant flow, farther from the motive power source.
U-tube manometer
A low-pressure measuring device that contains a liquid in a U-shaped tube. The level of the liquid on one side of the tube becomes lower as the pressure applied to it increases, while the level of the liquid on the other side of the tube becomes higher. The difference in the two levels is in proportion to the applied pressure. Differences in the liquid levels can be calibrated to give direct pressure readings.


Refer to volt-ampere.
vacuum pressure
Pressure below atmospheric pressure, similar to the suction side of a centrifugal refrigeration machine, or the design of a steam heating system, usually measured in inches of mercury.
The numerical value of any number without regard to sign.
The absence of matter, including air, in either an open or confined space.
valve, check
A valve that allows (fluid) flow in one direction only.
A device used for controlling or regulating the pressure or flow of a liquid or gas.
valve, pressure relief
A valve held closed by a spring or other means and designed to automatically relieve pressure in excess of its setting. Also called safety valve.
valve, reducIng
A valve that maintains a uniform pressure on its outlet side, irrespective of how the pressure on its inlet side may vary above the pressure to be maintained.
valve, reverse-actIng diaphragm
A type of valve that opens with the admission of fluid pressure to a diaphragm, and closes when the pressure is released.valve, solenoid
Usually an electrically actuated valve (consisting of a coil with a plunger), used to control the flow of a fluid. Also called a solenoid.
vapor barrier
A moisture-impervious layer applied to the surfaces enclosing a humid space, used to prevent moisture from traveling to a point where it may condense due to lower temperature.
vapor, superheated
Vapor at a temperature higher than the saturation temperature (i.e, the boiling point) at the existing pressure.
A condition or a factor that can be monitored, measured, and/or altered by a control, such as temperature, current, voltage, humidity, liquid level, pressure, etc.
A term given to a variable transformer that provides a voltage step-up or step-down. See also autotransformer.
A semiconductor device with voltage-sensitive resistance, commonly used as a lightning arrester. Overvoltages can be passed quickly, but operating voltages have no effect.
Refer to volt-ampere reactive.
Variable air volume systems.
A line used to represent both direction and magnitude. Used in calculating capacitive reactance, inductive reactance, and impedance in alternating current circuits.
velocity, outlet
The average discharge velocity of primary air being discharged from the outlet, normally measured in the plane of the opening.
velocity, room
The average sustained residual air velocity level in the occupied zone of the conditioned space (e.g., 65, 50, 35 ft/mm).
velocity, terminal
The highest sustained airstream velocity existing in the mixed air path at the end of the throw.
The process of supplying or removing air, by natural or mechanical means, to or from any space. Such air may or may not be conditioned.
very large-scale integration (VLSI)
A microcircuit generally considered to have more than 1,000 elements.
Volatile organic compounds. Chemical fumes given off by some man-made materials.
voice-grade line
A telephone communication channel that has a band width wide enough for voice broadcast.
volatile memory
A memory that will lose its information or data when power is removed.
volatile storage
A memory system that will lose all of its data when power is lost. Contrast with nonvolatile storage.
The unit of electrical potential. volt-amperes reactive (VAR) The power consumed by an inductive or capacitive circuit. It is derived by multiplying the voltage times the reactance times the power factor.
voltage drop
The difference in voltage between two points. The result of the loss of electrical energy as a current flows through a resistance.
The amount of potential difference across two points in a circuit. See also electromotive force.
volt-ampere (VA)
The unit of apparent power in an ac circuit in which the voltage and current are out of phase. One volt-ampere is one volt times one ampere.
An instrument used to measure the voltage in a circuit.
volume, specific
The volume of a substance per unit mass. The reciprocal of density volumetric efficIency Percent found by dividing the actual volume of gas pumped by a compressor by its displacement, both expressed in the same units (usually cfm).


water, make-up
Water supplied with intent to replenish for example, water replacing that lost by evaporation.
The unit of electric power. One horsepower equals 746 watts, 1 watt = 3.4 12 Btu per hour.
An instrument used to measure the actual or true power in an alternating current circuit.
1) A progressive, energy-bearing disturbance propagated from point to point in a medium. A physical activity (like ripples in a pond) that rises or falls periodically as it travels through a medium e.g., sound, light, heat, radio and TV signals, etc. 2) Electrical or electromagnetic radiation through a medium, or through a vacuum.
The distance between the corresponding valleys or peaks of adjacent waves.
wet-bulb temperature
The air temperature indicated by a thermometer with a wetted wick on the bulb.
A conductor, either bare or insulated, through which electrons can flow.
Restriction of area for a flowing fluid, causing a loss in pressure by (internal and external) friction without loss of heat or performance of work.
wire nut
A solderless connector for wires.
In computer terminology; a unit of information comprised of a certain number of bits in one location. In modern computers, a word is normally 16 bits, or two bytes.
A grid of column and rows upon which you manipulate data (values) and formulas to perform an analytical job.
To copy from one form of storage to another for example, to copy information from a hard disk to a floppy disk.
wye transformer
A transformer that has its windings connected together in the shape of a Y. In a three-phase alternating current circuit, this produces a balanced load.


Symbol for reactance. See also reactance.


The part of the flaring tool that fits over the block


Symbol for impedance. See also impedance.
Zener diode
A semiconductor device that controls or regulates the voltage in a circuit. Also called an avalanche diode.
zone control
A heating/cooling control method in which a building or large space is divided into two or more smaller areas, each of which is controlled by its own thermostat.
zone, comfort
Average: the range of effective temperatures over which the majority (50% or more) of adults feel comfortable. Extreme: the range of effective temperatures over which one or more adults feel comfortable.