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.Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA, 2004 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 (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.Heating and Cooling of Buildings, Kreider and Rabl, McGraw-Hill, New York, 1994 But for an instantaneous fuel consumption rate, the thermal efficiency may be better.
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. (accessed 2014-12-14) So from a theoretical perspective, in some use cases the name "efficiency" may be misleading.
+ Some typical AFUE numbers Fuel Furnace/boiler AFUE Heating oil Cast iron (pre-1970) 60% Retention head burner 70–78% Mid efficiency 83–89% Electric heating Central or baseboard 100% Geothermal heat pump see COP Air source heat pump see HSPF Natural gasStandard efficiency 78–84% Condensing 90–97% PropaneStandard efficiency 79–85% Condensing 88–95% Firewood Conventional 45–55% Advanced 55–65% State-of-the-Art 75–90%
See also Coefficient of performance (COP) Energy efficiency ratio (EER) Seasonal energy efficiency ratio (SEER) ASHRAE Air-Conditioning, Heating, and Refrigeration Institute (AHRI)
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