Furnace size for 2,500 square feet, by climate
The heat output the house needs, and the input rating of the furnace to buy at 96% and at 80% efficiency (AFUE). Standard furnace inputs are 40,000 to 140,000 BTU/hr.
| Climate | Rule of thumb | Output needed (BTU/hr) | Furnace at 96% AFUE | Furnace at 80% AFUE |
|---|---|---|---|---|
| Very hot (zone 1) | 25 BTU/ft² | 62,500 | 80,000 | 80,000 |
| Hot (zone 2) | 30 BTU/ft² | 75,000 | 80,000 | 100,000 |
| Warm (zone 3) | 35 BTU/ft² | 87,500 | 100,000 | 120,000 |
| Mixed (zone 4) | 40 BTU/ft² | 100,000 | 120,000 | 140,000 |
| Cool (zone 5) | 45 BTU/ft² | 112,500 | 120,000 | Over 140,000: two furnaces or a custom system |
| Cold (zone 6) | 50 BTU/ft² | 125,000 | 140,000 | Over 140,000: two furnaces or a custom system |
| Very cold (zone 7+) | 60 BTU/ft² | 150,000 | Over 140,000: two furnaces or a custom system | Over 140,000: two furnaces or a custom system |
Before you buy
These are rule-of-thumb figures. A contractor should do a Manual J load calculation, which accounts for windows, insulation and air leakage, because an oversized furnace short-cycles, wastes fuel and heats unevenly. Insulating the attic first can shrink the furnace you need: see the attic insulation calculator.
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Tools & materials for this project
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Frequently asked questions
What size furnace for 2,500 square feet?
In a mixed climate (40 BTU per ft²), about 100,000 BTU/hr of heat, which is a 120,000 BTU input furnace at 96% efficiency (140,000 at 80%).
How many BTUs to heat 2,500 square feet?
Between 62,500 BTU/hr in a very hot climate and 150,000 BTU/hr in a very cold one. Insulation, windows and ceiling height move it up or down.
Does a poorly insulated 2,500 sq ft house need a bigger furnace?
Yes: the estimate rises about 15%, to 115,000 BTU/hr in a mixed climate. Sealing and insulating first is usually cheaper than a bigger furnace.
Related sizes and guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.