What is AFUE?

Annual Fuel Utilization Efficiency (AFUE) is a measure of how effective a heating appliance (such as a furnace) converts fuel (namely natural gas or propane) into energy on a yearly basis. The Department of Energy (DOE) test procedure is defined as the "heat transferred to the conditioned space divided by the fuel energy supplied."

Therefore, the higher the AFUE the better.

The minimum AFUE rating that a heating unit must have as established by the government is 78%. However, to receive the Energy Star, a unit must have an AFUE of at least 90%. The current highest AFUE furnace is 97%.

All models that meet the Energy Star requirements are "condensing" units and use the unit's byproducts (or flue gas) to take advantage of the heat normally exhausted through the chimney.