Furnaces are mechanical pieces of equipment used for space heating. Furnaces consist of a burner with a combustion air intake, fuel intake and an igniter. The hot combustion flames are routed through a heat exchanger, where heat is exchanged to the cold air as it is blown across the heat exchanger coils. Warm air is then blown to the space and the combustion gases/products exit the furnace through an exhaust vent pipe.

The fuel that is most commonly used is natural gas. Furnaces can be used in both residential and commercial situations.


The two main types of furnaces are condensing and non-condensing furnaces. The traditional non-condensing furnace operates in the initial description of a furnace. These furnaces can have efficiencies in the range of 80% to 84% AFUE. A condensing furnace takes the combustion products that were initially routed to the exhaust vent and passes them through another heat exchanger. This extracts more heat to the air and cools the combustion products to a temperature where water begins to condense out of the air. Because of the water, this second heat exchanger is made of a corrosive resistive material. A condensing furnace can have efficiencies in the range of 90% to 98% AFUE.


The annual fuel utilization efficiency (AFUE) is the term used by manufacturers to rate the annual efficiency of their furnaces. It describes the ratio of the amount of useful heat out of the furnace compared to the amount of fuel input to the furnace. This efficiency rating is regulated by the Department of Energy (DOE) and is used to take into account the constant on/off operation and seasonal effects on the furnace. The DOE requires that all furnaces have efficiencies greater than 78% AFUE.

Steady state efficiencies are also provided by the manufacturer and indicate the best efficiency of the furnace when operated at peak conditions.