What Size Furnace Do I Need? BTU Chart by Square Feet and Climate Zone
Part of the project planners: Planning heating, cooling and ventilation and Getting the house ready for winter.
A 2,000 sq ft home in a mixed climate needs about 80,000 BTU per hour of heat, using 40 BTU per square foot. With a 96% efficient furnace that is 83,333 BTU/hr of input, so a 100,000 BTU/hr furnace. The figure per square foot depends on the climate zone, and rises for a poorly insulated house. The furnace size calculator works it out for your own home, zone, ceiling height and efficiency.
Heat needed by climate zone
For a 2,000 sq ft home with average insulation and 8 ft ceilings:
| Climate | Rule of thumb | Heat output needed |
|---|---|---|
| Very hot (zone 1) | 25 BTU per sq ft | 50,000 BTU/hr |
| Hot (zone 2) | 30 BTU per sq ft | 60,000 BTU/hr |
| Warm (zone 3) | 35 BTU per sq ft | 70,000 BTU/hr |
| Mixed (zone 4) | 40 BTU per sq ft | 80,000 BTU/hr |
| Cool (zone 5) | 45 BTU per sq ft | 90,000 BTU/hr |
| Cold (zone 6) | 50 BTU per sq ft | 100,000 BTU/hr |
| Very cold (zone 7 and up) | 60 BTU per sq ft | 120,000 BTU/hr |
Furnace size by home size and climate
The input rating to shop for, at 96% efficiency and average insulation (in BTU per hour). Above 140,000 a large house may need two furnaces or a custom system:
| Home size | Zone 1 (25) | Zone 3 (35) | Zone 4 (40) | Zone 6 (50) | Zone 7 (60) |
|---|---|---|---|---|---|
| 1,000 sq ft | 40,000 | 40,000 | 60,000 | 60,000 | 80,000 |
| 1,500 sq ft | 40,000 | 60,000 | 80,000 | 80,000 | 100,000 |
| 2,000 sq ft | 60,000 | 80,000 | 100,000 | 120,000 | 140,000 |
| 2,500 sq ft | 80,000 | 100,000 | 120,000 | 140,000 | over 140,000 |
| 3,000 sq ft | 80,000 | 120,000 | 140,000 | over 140,000 | over 140,000 |
| 4,000 sq ft | 120,000 | over 140,000 | over 140,000 | over 140,000 | over 140,000 |
Output, input and AFUE
For the 2,000 sq ft home in a mixed climate:
| Efficiency | Heat output | Input needed | Furnace to buy |
|---|---|---|---|
| 80% AFUE | 80,000 BTU/hr | 100,000 BTU/hr | 100,000 BTU/hr |
| 90% AFUE | 80,000 BTU/hr | 88,889 BTU/hr | 100,000 BTU/hr |
| 96% AFUE | 80,000 BTU/hr | 83,333 BTU/hr | 100,000 BTU/hr |
The heat the house needs (the output) is the same, but a less efficient furnace needs more input. A poorly insulated house (15% more) needs 92,000 BTU/hr of output.
Why not too big?
- Short cycling: an oversized furnace heats the house quickly and shuts off, which wastes fuel and wears the parts.
- Comfort: long, steady runs even out the temperature. Short blasts cause hot and cold swings.
- Noise and ducts: a big furnace pushes too much air through the ducts.
- Cost: a bigger furnace costs more to buy.
Before you replace a furnace
- Ask for a Manual J load calculation, and compare it with the old furnace, which may be oversized.
- Improve the insulation and seal the air leaks first: they can cut the size of furnace you need.
- Check the ducts for leaks and size: a good furnace in leaky ducts delivers little heat.
- Think about a heat pump, or a dual-fuel system, if electricity is cheap or gas is dear.
- Fit a carbon monoxide detector near the furnace and the bedrooms.
See the AC size calculator for cooling and the BTU conversion chart. This is an estimate for planning: a heating contractor should confirm the size.
Frequently asked questions
What size furnace do I need for a 2,000 square foot house?
About 80,000 BTU per hour of heat output in a mixed climate (40 BTU per sq ft), which is 83,333 BTU/hr of input at 96% AFUE, so a 100,000 BTU/hr furnace. In a cold climate you need more and in a warm one less.
How many BTU per square foot for a furnace?
About 25 to 30 BTU per square foot in the south, 35 to 45 in the middle of the country, and 50 to 60 in the far north, for an average house. Poorly insulated houses need more.
What is AFUE?
Annual fuel utilization efficiency: the percentage of the fuel that becomes heat. An 80% furnace turns 80% of the gas into heat, and a 96% condensing furnace 96%. The input rating is the output divided by the AFUE.
Is a bigger furnace better?
No. An oversized furnace turns on and off in short cycles, wastes fuel, wears out sooner and heats unevenly. The right size runs longer and is more comfortable.
Should I use this to size a new furnace?
It is a first estimate. A contractor should do a Manual J heat loss calculation for your house: it counts the insulation, the windows, the air leakage and the ducts, which change the answer a lot.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.