Definition
Annual Fuel Utilization Efficiency — measures furnace efficiency as a percentage of fuel converted to heat. Higher AFUE means lower gas bills.
What does an AFUE number actually tell you?
AFUE is a season-long average, not a lab peak: a 95% AFUE furnace turns 95 cents of every gas dollar into heat in your rooms, while an older 80% unit sends roughly 20 cents up the flue. Furnaces from the 1990s often test at 60–70% today because heat exchangers and burners degrade. The practical difference shows up on winter gas bills — upgrading from 80% to 95%+ trims fuel use for the same comfort.
Is a higher-AFUE furnace always the right upgrade?
Not automatically. Condensing (90%+) furnaces need a condensate drain and different venting, which adds install scope in some homes. And if the furnace is due for replacement anyway, it is worth pricing a heat pump at the same time — it heats and cools, and electricity in western Washington is comparatively clean and stable in price.
How do you check your furnace's AFUE?
Look at the nameplate inside the burner compartment or search the model number — the rating is published for every unit. Remember it's the as-new number: a 15-year-old 80% furnace with a tired heat exchanger and dirty burners delivers less. A combustion test during a tune-up measures what the furnace actually does today, which is the number worth making decisions with.
In the Puget Sound
Puget Sound heating seasons are long but mild — exactly the profile where efficiency differences compound month after month without the extreme peaks that stress equipment.
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Trade: Heating & Air