What is AFUE?
Plain-English explanation from a licensed Utah HVAC, plumbing, and electrical contractor.
AFUE is the gas-furnace efficiency rating — it tells you what percentage of the fuel you pay for actually becomes usable heat in your home versus what escapes up the flue.
Full Definition
AFUE (Annual Fuel Utilization Efficiency) is a percentage measuring how much of the natural gas a furnace burns over a full heating season is converted into useful heat for the home. The remainder is lost through combustion exhaust and standby losses. Federal minimum AFUE for new residential furnaces is 80% in Utah's non-weatherized northern zone, but most modern condensing furnaces achieve 95–98% AFUE by reclaiming heat from the flue gas via a secondary heat exchanger.
Upgrading from an 80% AFUE single-stage furnace to a 96% AFUE two-stage unit typically cuts Utah winter gas bills 15–18%. On a $1,200 annual heating bill that's $180–$215/year in savings.
Common Questions
Why don't 80% furnaces need a PVC flue?
80% AFUE furnaces produce hot, dry flue gas that won't condense in a metal vent. 90%+ condensing units cool the flue gas enough that water vapor condenses, requiring acid-resistant PVC venting and a drain for the condensate.
Will a 96% furnace work at Utah elevation?
Yes, but installers must derate the BTU input by roughly 4% per 1,000 ft above 2,000 ft elevation. A 100,000 BTU furnace in Park City (7,000 ft) effectively delivers ~80,000 BTU and must be sized accordingly.
Related Terms
Two-Stage HVAC
Two-stage equipment can run at full capacity OR a lower ~60% capacity — using low stage most of the time gives better comfort, quieter operation, and lower bills than a single-stage unit.
Modulating Furnace
A modulating furnace varies its gas input continuously from about 35% to 100% to exactly match your home's heat loss — instead of cycling on and off, it just runs longer at lower output.
Manual J Load Calculation
Manual J is the industry-standard whole-house heat-loss and heat-gain calculation that determines exactly how big your furnace and AC need to be — too big or too small both hurt comfort and efficiency.
Recent HVAC work in Utah
A few installs and service calls from the AYSP crew.
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