Furnace size calculator

Input against output

A furnace nameplate carries two BTU/h numbers. Input is how much fuel it burns per hour. Output is how much of that heat reaches the air. AFUE is the ratio, averaged over a heating season: an 80,000 BTU/h input furnace at 80% AFUE delivers about 64,000 BTU/h; the same input at 96% delivers about 76,800.

Size against the load in output, then divide by AFUE to get the input you have to order. Doing it the other way round — matching the load to the number on the box — buys a furnace about 20% too big every time.

What each input does

Floor area, zone and envelope set the heating load in output BTU/h — a screening load, so treat a borderline answer as a reason to run the Manual J worksheet. AFUE turns that output into the input you order. Altitude applies the published US derate: above 2,000 ft, input drops 4% for each 1,000 ft of elevation above sea level. The model's own installation manual governs, and the furnace sizing guide covers why, and the high-altitude kit that goes with it.

Sources: Mestek/Sterling, Separated Combustion Gas-Fired Indoor Duct Furnace installation and service manual (SCDFM-3) (read 2026-10-18)

Two worked examples

Both use the calculator's default house: 1,800 ft² in zone 5 with an average envelope, which it puts at 81,000 BTU/h of output.

At 5,280 ft the derate is 4% × 5.28 ≈ 21%, so the condensing furnace that fits at sea level steps up one nominal size. The dial shows how much spare capacity the pick leaves over the load — a large number there is the oversizing warning, not a safety margin.

Reading the airflow line

The result also prints the airflow the output needs at a 55 °F rise, the middle of a typical nameplate range (CFM = output ÷ (1.08 × rise)). If the ducts cannot move it, the rise climbs and the limit switch trips. Work the airflow at other rises, or back-calculate it from a measured rise, with the CFM per ton calculator, and check the duct side with the static pressure calculator before blaming the furnace.

What the standard requires

Non-weatherized gas furnaces manufactured today must be at least 80% AFUE. Furnaces manufactured on or after 2028-12-18 must be at least 95% AFUE, which means condensing — a second heat exchanger, PVC venting and a condensate drain. If you are quoting a replacement now in a house with a masonry chimney, that date is worth mentioning: the venting work is easier to do once.

• Zone 1
• Zone 2
• Zone 3
• Zone 4
• Zone 5
• Zone 6
• Zone 7

Sources: 10 CFR 430.32(e)(1)(iii) (read 2026-09-04)