AFUE
Annual fuel utilization efficiency: the share of the fuel energy a furnace turns into space heat across a whole heating season, written as a percentage. An 80 percent figure means four fifths of the fuel becomes useful heat and the rest leaves as flue loss and off-cycle loss.
Also called annual fuel utilization efficiency, furnace efficiency rating
What does AFUE mean on a furnace?
AFUE is the percentage of a furnace's seasonal fuel energy that becomes space heat rather than flue or standby loss. Mid-efficiency gas units sit near 80 percent and condensing units near 95 percent.
AFUE: how it works
A furnace test runs it through repeated cycles and totals the heat delivered to the ductwork against the heat in the fuel burned, including the energy lost up the flue while firing and the heat that escapes between cycles. A non-condensing furnace sends hot combustion gas outside and gives up the water vapour in it, which caps the figure in the low 80s. A condensing furnace adds a second heat exchanger that pulls heat back out of that gas until water condenses from it, which is where the extra points to the mid-90s come from. Electric resistance has no flue at all, so almost every unit of input becomes heat in the air.
Where you meet the term
- On the yellow energy label and the spec sheet of any furnace offered for sale.
- In a replacement quote, where an 80 and a 96 option are priced against expected fuel use.
- In a minimum-efficiency rule, where a region sets a floor AFUE for new gas furnaces.
How to check it
- Read it off the rating plate or the model number. The AFUE is printed on the cabinet label, and many model numbers carry it in the digits, such as a 96 in the middle of the string. Owner check
- Match the venting to the claim. A condensing furnace in the mid-90s vents through plastic pipe and produces condensate. A metal flue and no drain means a non-condensing unit in the 80s, whatever a listing says. Technician
AFUE: what it is not
- A high AFUE does not guarantee a low bill. It sets how much of the fuel is used, while the fuel price and the building's heat loss set the total.
- AFUE is not the running efficiency at any given moment. It is a seasonal average that already includes startup and off-cycle losses.
AFUE questions
Is a 96 AFUE furnace worth it over an 80?
It burns about a sixth less fuel for the same heat, so the payback depends on how cold the climate is and how expensive the fuel is. In a long, cold heating season the difference adds up faster.
Why is an electric furnace 100 percent AFUE but not cheap to run?
It converts nearly all its input to heat because there is no flue, but the input is electricity, which per unit of heat is often dearer than gas. Efficiency and running cost are separate questions.
Does AFUE apply to a heat pump?
No. A heat pump moves heat rather than burning fuel, so it is rated by coefficient of performance and HSPF2, which can exceed 100 percent equivalent because the output is more than the electrical input.
Where this definition comes from
Written from government guidance, manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Furnaces and boilersDepartment of Energy consumer guidance defining AFUE as the ratio of annual heat output to annual fuel energy consumed, expressed as a percentage over the whole heating season, with mid-efficiency gas furnaces at roughly 80 to 83 percent, high-efficiency condensing furnaces at roughly 90 to 98 percent, and electric resistance furnaces at close to 100 percent because there is no flue loss, alongside the note that a high AFUE does not by itself lower a bill where the fuel is expensive.
- Gas furnace technical frequently asked questionsRheem gas furnace technical FAQ: improper phasing or polarity and a dirty flame sense rod both preventing flame rectification, incorrect manifold gas pressure as a cause of failed flame sense, four ignition trials with a one-blink code reported after four failed trials or four flame failures in one call for heat, reset by removing the 115 volt supply or switching the thermostat off for five to ten seconds, the main limit opening on low airflow across the heat exchanger or excessive manifold gas pressure, and a pressure switch failing to close on a blocked vent, a damaged heat exchanger, or an induced draft blower not running to speed.