Furnace Efficiency & AFUE Savings Calculator

Estimate your annual heating cost savings when upgrading from an older furnace to a higher AFUE (Annual Fuel Utilization Efficiency) rated model.

Typical older furnaces: 56–80%. Check your furnace nameplate or manual.
High-efficiency furnaces: 90–98.5% AFUE. ENERGY STAR minimum: 95%.
Check your utility bills for the past 12 months of heating fuel usage.
Find this on your utility bill. U.S. avg: natural gas ~$1.20/therm, propane ~$2.50/gal, oil ~$3.80/gal, electric ~$0.13/kWh.
Include equipment + installation. Used to calculate simple payback period.

Formula

Step 1 – Current Annual Heating Cost:
Costcurrent = Usageannual × Priceper unit

Step 2 – New Annual Fuel Usage (same heat output, higher efficiency):
Usagenew = Usagecurrent × (AFUEcurrent / AFUEnew)
This works because: Heat delivered = Usage × AFUE. To deliver the same heat at a higher AFUE, less fuel is needed.

Step 3 – Annual Savings:
Savings = (Usagecurrent − Usagenew) × Price = Costcurrent × (1 − AFUEcurrent / AFUEnew)

Step 4 – Simple Payback Period:
Payback (years) = Installed Cost / Annual Savings

Example: Upgrading from 80% to 96% AFUE with 800 therms/year at $1.20/therm:
New usage = 800 × (0.80/0.96) = 666.7 therms/yr
Annual savings = (800 − 666.7) × $1.20 = $160/year (16.7%)

Assumptions & References

  • AFUE (Annual Fuel Utilization Efficiency) measures what percentage of fuel is converted to usable heat over a heating season, per DOE 10 CFR Part 430.
  • Calculation assumes the same annual heat load (BTUs delivered) before and after upgrade — only efficiency changes.
  • Fuel prices are assumed constant over the projection period. Actual savings will vary with fuel price changes.
  • Savings projections do not account for time value of money (not NPV-adjusted).
  • U.S. federal minimum AFUE standards: 80% for gas furnaces (non-weatherized), 83% for oil furnaces (DOE, 2015).
  • ENERGY STAR certified gas furnaces require ≥95% AFUE (ENERGY STAR Version 4.1).
  • Typical AFUE ranges: older furnaces 56–70%, mid-efficiency 80–83%, high-efficiency 90–98.5%.
  • Default fuel price references: U.S. EIA average residential prices (2023–2024).
  • Does not include installation costs, duct losses, or thermostat setback savings.
  • Federal tax credit: 30% of cost up to $600 for qualifying high-efficiency furnaces (IRA 2022, 25C credit — consult a tax advisor).

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