Annual fuel utilization efficiency

The annual fuel utilization efficiency (AFUE; pronounced 'A'-'Few' or 'A'-'F'-'U'-'E') is a thermal efficiency measure of space-heating furnaces and boilers. The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients.^[1] It is a dimensionless ratio of useful energy output to energy input, expressed as a percentage. For example, a 90% AFUE for a gas furnace means it outputs 90 BTUs of useful heating for every 100 BTUs of natural gas input (where the rest may be wasted heat in the exhaust). A higher AFUE means higher efficiency.

The method for determining the AFUE for residential furnaces and boilers is the subject of ASHRAE Standard 103. A furnace with a thermal efficiency (η_th) of 78% may yield an AFUE of only 64% or so, for example, under the standard's test conditions. When estimating annual or seasonal energy used by combustion devices, the AFUE is the better efficiency measure to use in the calculations.^[2] But for an instantaneous fuel consumption rate, the thermal efficiency may be better.

Note that the theoretical limit for a conventional furnace's instantaneous efficiency is 100%, whereas a heat pump used for building heating may exceed 100%. For example, a COP of 1.5 is equivalent to 150%. Heat pumps are readily available for electric and gas sources.^[3] So from a theoretical perspective, in some use cases the name "efficiency" may be misleading.