Furnace Input vs Output (AFUE) Calculator
A furnace’s BTU rating is usually its input. Multiply by the AFUE to see how much heat it actually delivers — and whether that is more than your home needs.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
Over 140 %
- Output ÷ heat loss
- 152%
- Heat output
- 76,000BTU/h
80,000 BTU/h of input at 95 % AFUE delivers about 76,000 BTU/h of heat over a season.
- ENERGY STAR’s HVAC design report recommends furnace output of 100–140 % of the design heat loss. This furnace is larger than that; a smaller or two-stage/modulating model may run more evenly.
- Input rating
- 80,000BTU/h
- Fuel energy not delivered (seasonal)
- 4,000BTU/h
Show the working
- Output = input × AFUE80,000 × 0.95 = 76,000 BTU/h
- Oversize check = output ÷ heat loss76,000 ÷ 50,000 = 152 %
Input, output and AFUE
AFUE (annual fuel utilization efficiency) is the share of the fuel’s energy that becomes usable heat over a typical heating season. Output = input × AFUE, so an 80,000 BTU/h input furnace at 95 % AFUE delivers about 76,000 BTU/h; at 80 % it delivers about 64,000 BTU/h.
Federal standards set a minimum of 80 % AFUE for non-weatherized gas furnaces today, rising to 95 % for units made from December 18, 2028.
Is it oversized?
Enter your design heat loss to compare. ENERGY STAR’s HVAC design report recommends furnace output between 100 % and 140 % of the design heat loss. Above that, a furnace runs in short bursts with larger temperature swings; two-stage and modulating furnaces soften this. No comparable published limit is applied to boilers here, though oversizing them causes short-cycling too.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- I know the
- Input rating (find the output)
- Input rating
- 80,000 BTU/h
- AFUE
- 95 %
- Equipment
- Furnace
- Design heat loss (optional, for the oversize check)
- 50,000 BTU/h
Result
- Output ÷ heat loss
- 152 %
- Heat output
- 76,000 BTU/h
- Input rating
- 80,000 BTU/h
- Fuel energy not delivered (seasonal)
- 4,000 BTU/h
80,000 BTU/h of input at 95 % AFUE delivers about 76,000 BTU/h of heat over a season.
Assumptions
- Output = input × AFUE. AFUE is a seasonal efficiency, so the steady output while firing can differ slightly from this figure.
- The furnace oversize guidance is ENERGY STAR’s recommended 100–140 % of the design heat loss (from its National HVAC Design Report, which takes it from ACCA Manual S).
Limitations
- Uses the heat loss you enter; a rule-of-thumb heat loss makes the oversize check meaningless.
- Two-stage and modulating furnaces are judged on their maximum output here.
Frequently asked questions
What does the “080” in my furnace model number mean?
Many manufacturers encode the input rating, so 080 often means 80,000 BTU/h input. Check the rating plate to be sure.
Is AFUE the same as the efficiency I measure with a combustion analyzer?
No. A combustion test measures steady-state efficiency at that moment. AFUE is a seasonal rating from a federal test that includes cycling losses.
My furnace is 200 % of my heat loss. Should I replace it?
Not just for that if it works well. When it is due for replacement, size the new one from a load calculation — usually much smaller.
Sources
- What is AFUE? — Carrier. AFUE is the share of fuel energy delivered as heat over a season.
- 10 CFR 430.32 — Energy and water conservation standards — U.S. Code of Federal Regulations (Cornell LII). Federal minimum furnace AFUE (80 % non-weatherized gas; 95 % for units made from 18 Dec 2028).
- National HVAC Design Report, Version 3/3.1/3.2 (Rev 13, historical) — ENERGY STAR (U.S. EPA). Equipment sizing limits from ACCA Manual S: cooling 90–115 % (single-speed), 90–120 % (two-speed), 90–130 % (variable-speed) recommended; furnace 100–140 % recommended.
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