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.
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).
The method for determining the AFUE for residential furnaces and boilers is the subject of ASHRAE Standard 103.
A furnace with a thermal efficiency (ηth) 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.