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. 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. But for an instantaneous fuel consumption rate, the thermal efficiency may be better.
| 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% |
Famous quotes containing the words annual, fuel and/or efficiency:
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—Unknown. Charlotte Observer (October 6, 1989)
“It is now many years that men have resorted to the forest for fuel and the materials of the arts: the New Englander and the New Hollander, the Parisian and the Celt, the farmer and Robin Hood, Goody Blake and Harry Gill; in most parts of the world, the prince and the peasant, the scholar and the savage, equally require still a few sticks from the forest to warm them and cook their food. Neither could I do without them.”
—Henry David Thoreau (1817–1862)
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—Baruch (Benedict)