Calculate how much you can save by upgrading to a higher AFUE furnace, including your new annual heating cost, fuel saved, percent savings, and upgrade payback period.
Furnace AFUE Savings Formula
AFUE (Annual Fuel Utilization Efficiency) is the percentage of the fuel your furnace burns that actually becomes usable heat for your home. The rest is lost, mostly up the flue. Because delivered heat equals fuel burned times AFUE, a more efficient furnace needs less fuel to deliver the same heat, and the savings follow directly from the two ratings:
AS = C * (1 - AFUEc / AFUEn)
If you know your fuel usage instead of your total cost, the calculator projects the new usage first and prices the difference:
Un = Uc * (AFUEc / AFUEn)
In payback mode, the annual savings are compared against what the new furnace costs to install:
P = IC / AS
Variables:
- AS is the annual heating cost savings in dollars
- C is the current annual heating cost in dollars
- AFUEc is the current furnace AFUE rating in percent
- AFUEn is the new furnace AFUE rating in percent
- Uc and Un are the current and new annual fuel use, in therms, ccf, gallons, or kWh
- P is the simple payback period in years
- IC is the installed cost of the new furnace in dollars
Pick what you want to solve for at the top of the calculator. In heating cost mode you enter your current annual heating cost and the two AFUE ratings, and the tool returns the annual savings, the new annual cost, and 5, 10, and 15 year totals. In fuel usage mode you enter annual usage and your price per therm, ccf, gallon, or kWh, and the tool also shows how many units of fuel you stop burning each year. In payback mode you add the installed cost of the new furnace, and the tool returns the simple payback period plus the net savings after the upgrade cost over the expected service life, which you can adjust under advanced options.
Savings by AFUE Upgrade and How to Read a Payback Period
This table shows the annual savings for every $1,000 you currently spend on heating fuel. The savings depend only on the ratio of the two AFUE ratings, so you can scale the numbers to your own bill. For example, if you spend $2,500 per year, multiply the value shown by 2.5.
| Current AFUE | To 90% AFUE | To 95% AFUE | To 98% AFUE |
|---|---|---|---|
| 60% | $333 | $368 | $388 |
| 70% | $222 | $263 | $286 |
| 80% | $111 | $158 | $184 |
| 85% | $56 | $105 | $133 |
| 90% | $0 | $53 | $82 |
Annual savings per $1,000 of current heating cost. Notice that the biggest step is getting out of a low efficiency furnace at all; going from 80% to 90% saves more than twice as much as going from 90% to 98%.
A savings number on its own does not tell you whether the upgrade is worth paying for. The payback period does. Use this guide to interpret the result from payback mode:
| Simple payback | What it means |
|---|---|
| Under 5 years | Strong case. Fuel savings recover the cost quickly, then keep accruing for the rest of the furnace’s life. |
| 5 to 10 years | Usually worthwhile, especially if the current furnace is past the midpoint of its 15 to 20 year life and a replacement is coming anyway. |
| 10 to 15 years | Marginal on fuel savings alone. Rebates, tax credits, comfort, and reliability need to carry part of the decision. |
| Over 15 years | Longer than a typical service life. The upgrade will likely never pay for itself in fuel unless prices rise sharply. |
Simple payback assumes flat fuel prices and no incentives, so treat it as a conservative baseline. Any rebate or credit shortens it in direct proportion.
Example Problems
Example 1: Annual savings from a heating bill
You spend $1,800 per year heating with an 80% AFUE furnace and are considering a 96% AFUE condensing model. First find the fraction of fuel spending you no longer need: 1 – 80/96 = 0.1667, or 16.67%. Then apply it to the bill: AS = $1,800 * 0.1667 = $300 per year. The new annual heating cost is $1,500, and over 15 years the savings total $4,500. Enter 80, 96, and 1800 in heating cost mode and the calculator returns the same numbers.
Example 2: Payback period on an upgrade
You heat with a 70% AFUE furnace for $2,000 per year, and a 95% AFUE replacement costs $6,500 installed. The savings fraction is 1 – 70/95 = 0.2632, so AS = $2,000 * 0.2632 = $526.32 per year. The payback is P = $6,500 / $526.32 = 12.35 years. With a 15 year service life set under advanced options, lifetime savings are $7,894.74, leaving $1,394.74 net after the installed cost. Per the table above, 12.35 years is marginal on fuel alone, so this upgrade makes the most sense if the old furnace is near the end of its life or rebates cut the installed cost.
Frequently Asked Questions
What AFUE rating does my current furnace have?
Newer furnaces list AFUE on the EnergyGuide label or in the model documentation. If the label is gone, age is a good proxy. Natural draft furnaces with a standing pilot light, common before the early 1990s, run about 56% to 70% AFUE. Mid efficiency fan assisted furnaces run 80% to 83%, and 80% has been the federal minimum for new non-weatherized gas furnaces for years. Condensing furnaces with a second heat exchanger and PVC vent pipes run 90% to 98.7%. If you are unsure, entering 70% for an old unit and 80% for a 2000s era unit brackets the realistic range.
Does a higher AFUE always mean a lower heating bill?
Only when you stay on the same fuel. AFUE measures how completely a furnace uses its fuel, not what the fuel costs. A 99% AFUE electric furnace can cost far more to run than an 85% AFUE natural gas furnace because electricity usually costs more per unit of heat than gas. This calculator assumes the same fuel and price for both furnaces, which is the right comparison for a like for like upgrade. If you are switching fuels, compare the cost per delivered BTU of each fuel first.
How does the 2028 federal furnace rule affect this decision?
A Department of Energy standard, upheld in court in late 2025, requires non-weatherized gas furnaces and mobile home gas furnaces manufactured after December 18, 2028 to be at least 95% AFUE. Existing furnaces can keep running and can still be repaired, but once the rule takes effect, replacement options will essentially all be condensing models. Practically, that means if you are pricing a replacement now, running this calculator with 95 or higher as the new AFUE previews the savings you would lock in either way, and an 80% unit installed today may be among the last of its kind.
