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.
| 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 | ||
| Standard efficiency | 78–84% | |
| Condensing | 90–97% | |
| Propane | ||
| Standard efficiency | 79–85% | |
| Condensing | 88–95% | |
| Firewood | Conventional | 45–55% |
| Advanced | 55–65% | |
| State-of-the-Art | 75–90% |
- ^ Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA, 2004
- ^ Heating and Cooling of Buildings, Kreider and Rabl, McGraw-Hill, New York, 1994
- ^ "Technical forms of E3 A". Archived from the original on 2014-03-23. Retrieved 2014-12-14. (accessed 2014-12-14)
- ^ Canada Mortgage and Housing Corporation, ed. (2008) [1993], A Guide to Residential Home Heating (PDF) (2008 ed.), Canadian Mortgage & Housing Corporation/Societe Canadienne d'Hpotheque et de Logement, ISBN 978-0-660-19848-4, archived from the original (PDF) on 2019-08-16, retrieved 2009-03-23