Understanding AFUE & why it matters

AFUE stands for Annual Fuel Utilization Efficiency. It’s a standardized rating that measures how efficiently a furnace or boiler turns fuel into heat for your home. 

AFUE for a furnace is like MPG for a car: the higher the number, the more efficiently it runs, and the less you’ll spend on fuel.

AFUE is always provided as a percentage, so for example, you might see a furnace advertised as having 80% AFUE. This means 80% of the fuel it burns is converted to usable heat, and 20% is wasted (out the exhaust or up the chimney).

“Fuel Utilization” is a key aspect of AFUE: it only applies to heating appliances that directly burn natural gas, propane, or oil. AFUE doesn’t apply to appliances that run on electricity.

Why AFUE rating matters

AFUE is used by the DOE to make sure all furnaces use the same benchmarking, so consumers can comparison shop for efficiency. Furnace and boiler manufacturers have to label their products with AFUE (the big yellow EnergyGuide sticker.)

DOE enforcement of AFUE standards also includes a minimum standard: as of 2025, new residential furnaces must have at least 80% AFUE. Currently scheduled for 2028, the new minimum will be 95% AFUE.

Use AFUE to reduce your gas bill

A higher AFUE can help you save money on energy bills. When fuel is used more efficiently, less fuel is burned to meet your heating requirements.

AFUE helps eco-conscious consumers

Homeowners who prioritize being environmentally friendly and reducing their carbon footprint should look for a furnace with the highest possible AFUE.

What is a good AFUE rating?

What makes an AFUE rating “good” depends on your goal:

Gas furnaces have different Energy Star criteria based on region because homes in colder northern states use their furnaces much more, so higher efficiency means they burn less fuel and save more over the long heating season.

AFUE can also be shorthand for understanding whether a home furnace has low, standard, or high efficiency:

*Since 2015, the minimum AFUE for a new furnace is 80%, so only older furnaces will typically have a low efficiency rating.

Learn more about standard-efficiency furnaces vs high-efficiency furnaces.

How important is AFUE when choosing a new furnace?

AFUE is a key factor when shopping for a new furnace, but it’s not the only thing that affects efficiency and performance. You should also consider:

• Ductwork: Leaky, poorly insulated, or overly complex air ducting can cause significant heat loss, especially for runs through unheated spaces like an unfinished basement or attic.
• Climate: The colder the region you live in, the more you will use your furnace, and the more you will save with a high-efficiency furnace. 
• Home insulation: If your home is better insulated, it will retain more heat, your furnace won’t have to work as hard, and you’ll burn less fuel.
• Maintenance: Keeping up with recommended preventive maintenance will keep your furnace running at the peak efficiency it is rated for.
• Size: A furnace that is too small or too big for your square footage will waste energy, no matter what the AFUE is. 
• Heating requirements: How warm you keep your house and whether you have a smart thermostat that maximizes efficiency make a difference in your energy bills, too.

Is a high-AFUE furnace worth it? In most cases, yes. But remember that your home’s overall efficiency depends on more than the furnace itself. Insulation, ductwork, and maintenance all work together to determine how much fuel you actually save.

FAQ

Here are helpful answers to other common questions homeowners have about AFUE.

What’s the difference between AFUE, SEER, UEF, and BTU?

AFUE, SEER, and UEF all measure efficiency. BTU measures how much heat is produced or removed. Here’s a breakdown:

BTU/h (British Thermal Units per hour) is also used to measure furnace capacity. For example, Navien’s NPF hydro-furnace comes in three sizes, 60,000, 80,000 and 100,000 BTU/h, which refers to the maximum heating input.

When the NPF700-100U5CH model is running at its highest setting, it ​will burn enough fuel to create 100,000 BTUs of heat energy every hour.

BTU is used for all appliances to measure how much heating or cooling they do, and it’s also used to calculate efficiency ratings. You might see “therms” on your gas bill: one therm equals 100,000 BTU.

How is AFUE calculated?

To come up with the AFUE rating, regulators use this formula: total yearly heat output divided by total yearly energy input (both in BTUs). Multiply that number by 100 to arrive at a percentage.

% AFUE = (Furnace heat output / Fuel input) x 100

What does condensing technology have to do with AFUE?

High-efficiency furnaces also referred to as condensing furnaces are designed to capture extra heat from exhaust gases that older, non-condensing models simply release outdoors. They have a second heat exchanger to extract that additional  heat back into your home instead of wasting it. That’s why condensing furnaces achieve higher AFUE ratings, because they use more of the fuel you pay for.

Does an electric furnace have AFUE?

No. AFUE applies only to home heating systems that burn fuel like natural gas, propane, or oil. Electric furnaces don’t burn fuel; they convert electricity directly into heat. 

How can I tell what AFUE rating my furnace has?

Start by checking the big yellow EnergyGuide sticker on the outside of your furnace. Most modern furnaces display the AFUE rating on it. If you don’t see an EnergyGuide label, look for a metal nameplate or data tag on the inside of the furnace cabinet. You can also search the model number online or on the manufacturer’s website to find the specifications. Older furnaces without a label probably have AFUE below 80%.

Next steps

AFUE gives you an easy way to compare how efficiently furnaces use fuel, but it’s only part of the picture. Pairing a high-efficiency furnace with good insulation, sealed ducts, and regular maintenance is the best way to keep your home warm and your energy use low.

If you’re deciding what type of furnace to buy next, you might like our guides on single-stage vs. two-stage vs. variable-capacity furnaces and what to expect during a furnace installation. Both include practical information that affects comfort, efficiency, and long-term performance once you’ve decided on an AFUE range.