What is AFUE? The furnace efficiency rating, explained
AFUE is a percentage on a furnace label. It looks simple, but it only answers one question, and knowing which one keeps you from overpaying for the wrong thing.
AFUE in one sentence
AFUE stands for Annual Fuel Utilization Efficiency. It is the share of a furnace's fuel that ends up as heat in your home over a typical season. The rest goes up the vent.
A furnace rated 80% AFUE turns about 80% of its fuel into useful heat. The other 20% leaves as exhaust. A 95% unit loses only about 5%.
What do the AFUE numbers mean in practice?
Furnaces fall into two broad groups.
| Type | Typical AFUE range | What sets it apart |
|---|---|---|
| Standard (non-condensing) | Around 80% | Hot exhaust goes up a metal flue |
| High-efficiency (condensing) | 90% and above | A second heat exchanger pulls more heat out of the exhaust |
Condensing furnaces cool the exhaust so much that water vapor condenses. That is why they vent through plastic pipe and need a condensate drain.
What AFUE does not tell you
AFUE measures the furnace itself. It says nothing about the house around it. Leaky ducts, a poorly insulated attic or a badly sized unit can waste far more heat than the gap between two AFUE ratings.
It also does not compare fuels. A gas furnace at 95% AFUE and a heat pump are measured on different scales. A heat pump moves heat instead of burning fuel, so it uses ratings like HSPF2 and COP. Do not set the numbers side by side as if they were the same thing.
Finally, it is a seasonal average. Real results depend on how your system runs, how cold your winters are and how well it is maintained.
How to use AFUE when you shop
Use it to compare furnaces that burn the same fuel. Higher AFUE means less fuel wasted, so the unit can cost less to run each season.
But more efficient is not always the right buy. If you rarely run the heat, the savings from a higher rating may take a very long time to matter. If winters are long, they add up faster. Take a side based on how much you actually use the furnace.
Size matters as much as the number. An oversized furnace short cycles, which wastes fuel whatever its rating. Have a licensed contractor do a load calculation before you pick a unit.
Where do you find your furnace's AFUE?
Look at the yellow EnergyGuide label if it is still attached, or the nameplate inside the cabinet. The model number also lets you look it up on the manufacturer's site. Leave burner and gas-valve work to a licensed technician.
If you smell gas or a CO alarm sounds, leave the house and call 911 or the gas company from outside.
This article does not quote prices, rebates or tax credits. They change and vary by state and utility, so ask a local licensed contractor.
Frequently asked questions
What is a good AFUE rating?
Around 80% is standard for a non-condensing gas furnace, and 90% or higher is high-efficiency. Which is right depends on how cold your winters are and how much you run the heat.
Does a higher AFUE always mean lower bills?
Not always. It means less fuel wasted by the furnace itself. Leaky ducts, poor insulation or an oversized unit can cancel out the difference.
Can I compare AFUE with a heat pump rating?
No. AFUE measures fuel burned, while heat pumps move heat and use ratings like HSPF2 and COP. They are not on the same scale.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Energy Savings at Home: Heating & Cooling
- US EPA. Section 608 of the Clean Air Act