Efficiency Ratings
AFUE Rating Explained: 80% vs 90–98% Furnaces and What You Save
AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace's fuel energy that ends up as heat in your home over a typical year. An 80% furnace sends about a fifth of its fuel up the flue; 90–98% condensing furnaces recover heat from the water vapor in the exhaust and vent through plastic pipe. The higher the AFUE, the less gas or oil you burn for the same warmth.
What AFUE measures
AFUE compares the heat a furnace or boiler delivers over a modeled heating season with the fuel energy it consumes, including losses while starting, stopping and sitting idle.
AFUE = Annual heat delivered to the home ÷ Annual fuel energy consumed × 100%
Fuel needed = Heat needed ÷ AFUE
Two things are left out. AFUE does not count the electricity for the blower and inducer fan, and it stops at the furnace cabinet, so heat lost from leaky ducts in an attic or crawlspace is not included. A 96% furnace feeding ducts that lose 20% of their heat delivers far less than 96% of its fuel to your rooms.
80% vs 90–98% furnaces
| Standard 80% furnace | High-efficiency 90–98% furnace | |
|---|---|---|
| Heat exchangers | One primary heat exchanger | Primary plus a secondary (condensing) heat exchanger |
| Flue gas temperature | Hot, typically several hundred °F | Cool, often around 100–130 °F |
| Venting | Metal B-vent or lined masonry chimney | PVC, CPVC or polypropylene pipe to a side wall or roof |
| Combustion air | Usually drawn from the room | Often sealed combustion, drawing outdoor air through a second pipe |
| Condensate | None in normal operation | Several gallons a day in cold weather; needs a drain, sometimes a pump and a neutralizer |
| Typical installed cost | Lower | Higher, especially when venting must be rerouted |
A condensing furnace cools its exhaust below the dew point of the water vapor produced by burning gas. Condensing that vapor releases its latent heat into your airstream instead of sending it out the chimney, which is where most of the extra 10–18 points of efficiency come from. The condensate is mildly acidic, so it must go to a suitable drain, and the furnace needs freeze protection for its drain lines.
Cost impact: what each AFUE level burns
The table assumes a home needing 60 million BTU (600 therms) of delivered heat per year, typical of a mid-sized house in a cold-to-mixed climate, and natural gas at $1.40 per therm. Both numbers vary widely; substitute your own.
| AFUE | Gas burned per year | Annual cost at $1.40/therm | vs 80% |
|---|---|---|---|
| 60% (old, pilot-light era) | 1,000 therms | $1,400 | +$350 |
| 80% | 750 therms | $1,050 | baseline |
| 90% | 667 therms | $933 | −$117 |
| 95% | 632 therms | $884 | −$166 |
| 98% | 612 therms | $857 | −$193 |
In a northern home that needs 100 million BTU a year, the gap between 80% and 95% grows to about 197 therms, or roughly $276 a year at the same price. In a mild southern climate using 25 million BTU, it shrinks to about $69. That is why the case for a condensing furnace is strongest where winters are long and gas is expensive. Run your own numbers in the heating cost calculator.
Minimum AFUE rules
The long-standing US minimum for non-weatherized gas furnaces (the indoor kind most homes have) is 80% AFUE, and 81% for weatherized rooftop units. DOE finalized a rule in 2023 that would raise most new non-weatherized gas furnaces to 95% AFUE from late 2028, though that rule has faced political and legal challenges, so check its current status before planning around it. Canada already requires condensing efficiency for most new gas furnaces. ENERGY STAR furnaces are typically 95% AFUE or more in the northern US.
Estimating your own savings
To estimate what a new furnace would save, start from your gas bills. Add up the therms used in the heating months, subtract a typical summer month's use multiplied by the number of heating months (to remove water heating and cooking), and you have your furnace's annual fuel use. Then:
New fuel use = Old fuel use × Old AFUE ÷ New AFUE
Annual savings = (Old fuel use − New fuel use) × Price per therm
Example: a house that burned 800 therms of heating gas with a 78% furnace would burn about 800 × 0.78 ÷ 0.96 = 650 therms with a 96% furnace. At $1.40 a therm, that is about $210 a year. If the old furnace's AFUE is unknown, use the label guidance at the end of this page.
Choosing between 80% and condensing
- Cold climate, long heating season: a 95–97% furnace usually pays for its premium within its service life.
- Mild climate or mostly cooling: the fuel savings may be small; an 80% furnace, or a heat pump with or without gas backup, can make more sense. See gas vs electric heating.
- Venting: if a new condensing furnace leaves a gas water heater alone in an oversized masonry chimney, that chimney may need a liner so the water heater still drafts safely. Include this in the quote.
- Size: a higher AFUE doesn't fix an oversized furnace. Check the output with the furnace sizing calculator.
Gas piping, venting, combustion air and heat exchanger inspection are jobs for a licensed HVAC or gas contractor. Poor venting or a cracked heat exchanger can release carbon monoxide. Keep working CO alarms on every level of the home.
Reading AFUE on a furnace label
AFUE appears on the yellow EnergyGuide label and in the model's AHRI listing. Two-stage and modulating furnaces have similar AFUE to single-stage condensing units but run longer at low fire, which evens out temperatures and can reduce blower cycling. If your furnace is old enough that you can't find a rating, assume 56–70% for a unit with a standing pilot and no inducer fan, and around 80% for one with an inducer fan and metal vent.
Frequently asked questions
Is a 96% AFUE furnace worth it over 80%?
In cold climates usually yes: it burns about 17% less gas for the same heat. Where heating bills are small, the higher purchase and venting cost may not pay back.
Can I replace an 80% furnace with a 95% one in the same spot?
Usually, but the venting changes to plastic pipe through a wall or roof, a condensate drain is needed, and any water heater sharing the old chimney must be checked for safe venting.
Does AFUE apply to electric furnaces?
Electric resistance furnaces are close to 100% AFUE because there is no flue loss, but electricity per BTU usually costs much more than gas. A heat pump delivers two to four times more heat per kWh.
Does a furnace keep its AFUE as it ages?
The rating describes the furnace as built, but poor maintenance lowers real-world performance. Dirty burners, a clogged condensate trap, a restricted filter or incorrect gas pressure all reduce delivered heat. An annual check by a qualified technician keeps it close to its rating.
Last reviewed 2026-10-28