AFUE (Annual Fuel Utilization Efficiency)
The percentage of a furnace's fuel that becomes usable heat over a year. A 95% AFUE furnace turns 95 cents of every fuel dollar into heat; the rest leaves through the vent. Modern high-efficiency (condensing) furnaces run 90–98% AFUE, while many older units run 80% or below.
The longer answer
AFUE is a seasonal average, not a peak-efficiency snapshot — it accounts for startup losses, standby heat, and off-cycle jacket losses over a full heating season. That's why a 95% AFUE unit isn't five times better than an 80% unit; the real-world gap is roughly 15–19% less fuel for the same heat output.
The 80-vs-95 decision in Seattle usually hinges on how long you'll stay and how much you run the furnace. A condensing upgrade costs more upfront (PVC venting, condensate drain) but compounds across our long heating season. If you're replacing anyway and plan to stay 10+ years, 95% AFUE is the default recommendation — see the furnace efficiency comparison guide for the break-even math.
AFUE only applies to combustion equipment — heat pumps use HSPF2 and SEER2 instead. When comparing a new gas furnace to a heat pump conversion, you're comparing different fuel types and different efficiency metrics; the operating-cost comparison guide handles that math.
The Seattle angle
With Seattle-length heating seasons, the fuel savings from a higher-AFUE furnace add up faster than in milder climates.
Guides that use this term
Where this decision happens
Nearby in the glossary
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