Gas- and oil-fired furnaces provide warm, even heat throughout your home by circulating heated air through ducts.
The heat is created by burning the fuel (gas or oil) inside your furnace. (Some furnaces use outside air to help burn the fuel; others use air from inside your home.) When the fuel burns, the hot gases that are created go through curved metal tubing called a heat exchanger and then out of your home through a metal or plastic vent pipe. At the same time, the air that circulates through your ducts passes over the outside of the heat exchanger and takes on the heat from the hot metal. The warm air is then circulated through your home. (By keeping combustion air and supply air separate, the heat exchanger allows the air in your home to be heated without contaminating it with the toxic byproducts of combustion.)
Some furnaces offer additional features that provide greater comfort (as well as additional energy savings). Two-speed furnaces can run on low speed up to 80% of the time, so they operate more quietly and run for longer periods of time than single-speed furnaces. Longer operating periods translate into fewer on/off cycles, fewer drafts and much smaller temperature swings -- only one or two degrees instead of the four-degree swings common with single-speed furnaces. Plus, better air circulation helps prevent air "stratification" warm air rising to the ceiling and cold air settling on the floor. In short, you get consistent, even heat throughout your home.
Variable-capacity furnaces provide the ultimate combination of comfort, efficiency and quiet performance. In addition to the benefits of two-speed furnaces, they offer "smart" motors than can monitor your home’s comfort needs and automatically adjust the volume and speed of air to provide the most efficient heating or cooling. They offer added electrical efficiency as well: the "smart" fan motors on variable-capacity furnaces use less electricity than a 100-watt light bulb. They operate so efficiently that they can actually increase the efficiency rating of your central air conditioning system and offer you added energy savings when you use continuous fan operation in any season.
If you'd like to know more about topics like efficiency, go to U.S. Department of Energy.