What is AFUE? Furnace efficiency explained
AFUE (Annual Fuel Utilization Efficiency) measures what fraction of a furnace's or boiler's fuel becomes useful heat over a season. A 95% AFUE condensing furnace turns 95 cents of every fuel dollar into heat; an older 80% unit wastes 20% up the flue. It is the number we divide fuel cost by when comparing against a heat pump.
What AFUE means
AFUE is a percentage from 0-100. A 95% gas furnace delivers 95% of the fuel's energy as heat; the rest is lost in combustion and flue gases. Modern condensing furnaces reach 90-98% AFUE; older atmospheric units are 78-83%.
How AFUE affects running cost
In our cost formula, fuel needed = heat demand divided by AFUE. Dropping from 95% to 80% AFUE raises fuel use by about 19% for the same heat - a direct hit to your bill. AFUE does not count electricity for the blower or distribution losses in ducts.
Typical AFUE by fuel
This site assumes 95% for high-efficiency natural gas, 92% for condensing propane, and 85% for a typical oil boiler/furnace. You can change all of these in the calculator.
| System | Typical AFUE |
|---|---|
| Condensing gas furnace | 90-98% |
| Standard/older gas furnace | 78-83% |
| Condensing propane furnace | 90-95% |
| Oil boiler/furnace | 80-90% |
| Electric resistance furnace | 100% (but COP 1.0) |
Source: Industry references (AHRI/DOE/ENERGY STAR). Data as of June 2026.
Frequently asked questions
Is a higher AFUE always cheaper?
It lowers fuel use, but a 95% furnace costs more to buy than an 80% one. Whether the fuel savings pay back depends on your climate and fuel price. On running cost alone, higher AFUE always wins.
Does AFUE include electricity?
No. AFUE only measures combustion/heat-extraction efficiency. The furnace blower and controls use electricity that AFUE ignores.
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Last updated: 2026-06-29