FURNACE SIZE
About this calculator
This furnace size calculator returns the heating output a residential furnace needs for a given conditioned area and climate zone. The rule of thumb runs 25-60 BTU per square foot of heating output depending on climate severity, building tightness, and insulation level. The calculator returns both heating output BTU/hr (what the equipment delivers to the house) and input BTU/hr (the rated nameplate size, accounting for typical 80-95% AFUE efficiency). For tight specs in cold climates or large houses, an HVAC contractor should run an ACCA Manual J load calculation against the actual building envelope.
How to use this calculator
Enter the conditioned floor area in ft² (heated space only — exclude unheated garage, attic, basement). Pick your climate zone — the calculator uses BTU/ft² heat-load multipliers ranging from 30 (mild) to 60 (very cold).
Pick the furnace efficiency (AFUE — Annual Fuel Utilization Efficiency). 80% is standard non-condensing (older units, simpler design); 90-95% are condensing furnaces with secondary heat exchangers and PVC venting; 96-98% are premium condensing models. Higher AFUE costs more upfront but saves 15-25% on annual heating bills.
The calculator returns output BTU (delivered to the house), input BTU (nameplate rating), and the next standard furnace size to buy.
Worked example
For a 2,000 ft² home in Chicago (cold climate, 50 BTU/ft²) with a 95% AFUE condensing furnace:
Output BTU: 2,000 × 50 = 100,000 BTU/hr. Input BTU at 95% AFUE: 100,000 ÷ 0.95 = 105,263 BTU/hr.
Next standard size: 120,000 BTU input. (Standard sizes: 40K, 60K, 80K, 100K, 120K, 140K.)
At $2,500-$5,000 for a 120K BTU 95% AFUE furnace, plus $1,500-$3,500 install: total $4,000-$8,500 for a basic upgrade replacing an existing 80% unit.
For the same 2,000 ft² home in Atlanta (mild, 30 BTU/ft²): output 60,000 BTU, input 63,000 → 80,000 BTU furnace. About half the heat-load of Chicago because the design temp is 50°F warmer.
For a 3,500 ft² very-cold-climate home (Minnesota, 60 BTU/ft²): output 210,000, input 221,000 → engineered (>140,000 BTU). Very large or very cold homes often need two furnaces or a hydronic boiler instead.
Common mistakes & waste factors
Confusing input and output BTU. Equipment is sold by input BTU (the gas burner rating). Output BTU is what actually heats the house. A 100K input furnace at 80% AFUE delivers 80K output; at 95% AFUE it delivers 95K. Always match output to load.
Oversizing for "headroom." Oversized furnaces short-cycle, never running long enough to fully heat the structure or temper the air. Result: cold spots, fast wear, higher fuel bills than right-sized.
Ignoring duct losses. Forced-air ducts in unconditioned attics or crawlspaces lose 15-30% of heat. The calculator gives net house load — equipment sizing must add duct losses.
Picking 80% AFUE because it's cheaper. The 95% AFUE upgrade typically pays back in 3-5 years on Northern winters through fuel savings — and resale value is higher.
Rules of thumb
Climate factor: mild 30 BTU/ft², moderate 40, cool 45, cold 50, very cold 60.
Standard furnace input sizes: 40K, 60K, 80K, 100K, 120K, 140K BTU/hr.
AFUE: 80% (older standard non-condensing), 90% (entry condensing), 95% (high-efficiency), 98% (premium).
Input BTU = Output BTU ÷ AFUE.
Duct losses: 15-30% of equipment capacity in unconditioned space. Ducted equipment must size for net load + duct loss.
Venting: 80% AFUE uses metal B-vent or atmospheric chimney. 90%+ uses PVC sidewall vent (cheaper to install, no chimney needed).
Common questions
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