AFUE (Annual Fuel Utilization Efficiency) measures how much of the fuel a furnace burns actually becomes useful heat. An 80% AFUE furnace turns 80 cents of every fuel dollar into heat; the other 20 cents goes up the flue. A 96% AFUE condensing furnace captures heat that would otherwise be lost.
AFUE tiers
- 80% AFUE — federal minimum, single-stage standard-efficiency. Metal flue venting. No rebates.
- 90-96% AFUE — condensing high-efficiency. PVC venting. Qualifies for Xcel + federal rebates.
- 97-98% AFUE — premium modulating or variable-speed. Highest rebate tier.
At Colorado altitudes, real-world efficiency drops slightly because thinner air affects combustion — about 4% per 1,000 feet of elevation. The relative ranking between units stays the same, but absolute BTU output is reduced.
Related: SEER, HSPF. See also: furnace installation.
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Right Way Mechanical is a family-owned HVAC contractor serving Wheat Ridge, Arvada, west Denver, and the Colorado foothills.