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:
“No annual training or muster of soldiery, no celebration with its scarfs and banners, could import into the town a hundredth part of the annual splendor of our October. We have only to set the trees, or let them stand, and Nature will find the colored drapery,—flags of all her nations, some of whose private signals hardly the botanist can read,—while we walk under the triumphal arches of the elms.”
—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)
““Never hug and kiss your children! Mother love may make your children’s infancy unhappy and prevent them from pursuing a career or getting married!” That’s total hogwash, of course. But it shows on extreme example of what state-of-the-art “scientific” parenting was supposed to be in early twentieth-century America. After all, that was the heyday of efficiency experts, time-and-motion studies, and the like.”
—Lawrence Kutner (20th century)