Annual fuel utilization efficiency

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The annual fuel utilization efficiency (AFUE; pronounced 'A'-'Few' or 'A'-'F'-'U'-'E') is a thermal efficiency measure of combustion equipment like furnaces, boilers, and water heaters. 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 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.

Some typical AFUE numbers[3]
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 gas Conventional 55–65%
Mid-efficiency 78–84%
Condensing 90–97%
Propane Conventional 55–65%
Mid-efficiency 79–85%
Condensing 88–95%
Firewood Conventional 45–55%
Advanced 55–65%
State-of-the-Art 75–90%

See also[edit]

References[edit]

  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. ^ Canada Mortgage and Housing Corporation, ed. (2008) [1993], A Guide to Residential Home Heating (2008 ed.), ISBN 978-0-660-19848-4, retrieved 2009-03-23