Most of North American households rely on a central furnace to supply heat. Furnace works by blowing heated air through ducts that carry the hot air to rooms all over the home via air registers or grills. This kind of heating system is called a ducted warm-air system. It can be motorized by electricity, natural gas, or any fuel oil.

Inside furnace, the fuel is merge with air and burned. The flames warm a metal heat exchanger where the heat is converted to air. Air is forced through the heat exchanger by the “air handler’s” furnace fan and then forced through the ductwork downstream of the heat exchanger. At the furnace, burning products are vented out of the building through a flue pipe. Condensing” furnaces are made to regain much of this escaping heat by cooling exhaust gases well below 140°F, where water vapor in the exhaust condense into water. This is the main feature of a high-efficiency furnace (or boiler). These classically vent through a sidewall with a plastic pipe.

The currently furnace have not been updated since 1987. New furnace principles are presently under development by the U.S. Department of Energy.