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:
“I would not have every man nor every part of a man cultivated, any more than I would have every acre of earth cultivated: part will be tillage, but the greater part will be meadow and forest, not only serving an immediate use, but preparing a mould against a distant future, by the annual decay of the vegetation which it supports.”
—Henry David Thoreau (1817–1862)
“I had an old axe which nobody claimed, with which by spells in winter days, on the sunny side of the house, I played about the stumps which I had got out of my bean-field. As my driver prophesied when I was plowing, they warmed me twice,—once while I was splitting them, and again when they were on the fire, so that no fuel could give out more heat. As for the axe,... if it was dull, it was at least hung true.”
—Henry David Thoreau (1817–1862)
“I’ll take fifty percent efficiency to get one hundred percent loyalty.”
—Samuel Goldwyn (1882–1974)