Furnace Efficiency & AFUE Savings Calculator

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%)