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 (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.[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.

Some typical AFUE numbers[4]
FuelFurnace/boilerAFUE
Heating oil Cast iron (pre-1970)60%
Retention head burner70–78%
Mid efficiency83–89%
Electric heating Central or baseboard100%
Geothermal heat pumpsee COP
Air-source heat pumpsee HSPF
Natural gas
Standard efficiency78–84%
Condensing90–97%
Propane
Standard efficiency79–85%
Condensing88–95%
Firewood Conventional45–55%
Advanced55–65%
State-of-the-Art75–90%

See also

References
  1. Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA, 2004
  2. Heating and Cooling of Buildings, Kreider and Rabl, McGraw-Hill, New York, 1994
  3. "Archived copy". Archived from the original on 2014-03-23. Retrieved 2014-12-14.CS1 maint: archived copy as title (link) (accessed 2014-12-14)
  4. Canada Mortgage and Housing Corporation, ed. (2008) [1993], A Guide to Residential Home Heating (PDF) (2008 ed.), ISBN 978-0-660-19848-4, retrieved 2009-03-23


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