Annual fuel utilization efficiency is the method of rating gas furnaces, according to the U.S. Department of Energy. The Federal Trade Commission requires all new furnaces, regardless of fuel type, to display an AFUE rating to allow consumers to make informed decisions.
The AFUE is a ratio that compares the amount of heat a furnace produces to heat energy available in the fossil fuel it consumes. In gas furnaces, most of the energy not converted to heat escapes through the flue as the products of incomplete combustion.
The AFUE does not consider the efficiency of the duct system in delivering heat. Up to 35 percent of the heat the furnace produces is lost if uninsulated ducts are located in unheated spaces, reports the U.S. Department of Energy.
As of 2015, the minimum AFUE for nonweatherized gas furnaces, including those for mobile homes, is 80 percent. Weatherized gas furnaces have a minimum AFUE rating of 81 percent.
Furnaces with heavy heat exchangers, open-air fuel flow and standing pilot lights are low-efficiency heating systems, with an AFUE of 56 to 70 percent efficiency. Medium-efficiency systems have an electronic ignition and blower to improve the fuel-to-air ratio, increasing the efficiency to between 80 and 83 percent. High-efficiency systems have a sealed combustion box and secondary combustion chamber. These systems offer an efficiency rating between 90 and 98.5 percent.