Furnace size chart
Recommended furnace input rating for a home with 8 ft ceilings and average insulation. The first figure is for an 80% AFUE furnace, the second for a 96% AFUE furnace.
| Home size | Hot IECC 1–2 | Warm IECC 3 | Mixed IECC 4 | Cold IECC 5 | Very cold IECC 6–7 |
|---|---|---|---|---|---|
| 1,000 sq ft | 40k / 40k | 40k / 40k | 40k / 40k | 60k / 40k | 60k / 60k |
| 1,500 sq ft | 40k / 40k | 60k / 60k | 60k / 60k | 80k / 60k | 90k / 70k |
| 2,000 sq ft | 60k / 60k | 70k / 60k | 80k / 70k | 100k / 80k | 120k / 100k |
| 2,500 sq ft | 70k / 60k | 90k / 70k | 100k / 80k | 120k / 100k | 150k / 120k |
| 3,000 sq ft | 80k / 70k | 100k / 90k | 120k / 100k | 150k / 120k | 2 units / 150k |
Furnaces are commonly sold in 40,000, 60,000, 70,000, 80,000, 90,000, 100,000, 110,000, 120,000, 135,000 and 150,000 BTU input sizes. Some brands offer other steps.
Input vs output BTU
The number on a furnace's model tag is its input: how much fuel it burns per hour. The heat that reaches your rooms is the output, equal to input multiplied by AFUE (annual fuel utilization efficiency).
| Input rating | 80% AFUE output | 92% AFUE output | 96% AFUE output |
|---|---|---|---|
| 40,000 BTU | 32,000 | 36,800 | 38,400 |
| 60,000 BTU | 48,000 | 55,200 | 57,600 |
| 80,000 BTU | 64,000 | 73,600 | 76,800 |
| 100,000 BTU | 80,000 | 92,000 | 96,000 |
| 120,000 BTU | 96,000 | 110,400 | 115,200 |
This is why replacing an old 80% furnace with a 96% model often allows a smaller input size: the new unit delivers the same heat while burning less gas.
How the size is calculated
- Heat loss. The calculator estimates how much heat your home loses per hour on a design day, the outdoor temperature that is colder than 99% of hours in a typical year. It uses 18 BTU per sq ft in hot climates up to 38 BTU per sq ft in very cold climates, adjusted for ceiling height and insulation.
- Safety margin. It adds 20%, which covers morning recovery after a thermostat setback. ACCA Manual S allows furnace output up to 140% of the heating load, so a 20% margin keeps the furnace well within that limit.
- Input size. It divides the required output by the AFUE you choose and picks the smallest standard input size that delivers it.
| Climate zone | Design temp | Heat loss BTU/sq ft | With 20% margin |
|---|---|---|---|
| Hot · IECC 1–2 | 35 °F | 18 | 21.6 |
| Warm · IECC 3 | 25 °F | 22 | 26.4 |
| Mixed · IECC 4 | 15 °F | 26 | 31.2 |
| Cold · IECC 5 | 5 °F | 32 | 38.4 |
| Very cold · IECC 6–7 | −10 °F | 38 | 45.6 |
Problems with an oversized furnace
Many older homes have furnaces 50–100% larger than they need. An oversized furnace heats the house in short bursts, which causes uneven temperatures between rooms, more noise from high airflow, and more wear on the heat exchanger and igniter. It also may need larger ducts than the house has, which raises static pressure and shortens blower life.
If your old furnace kept the house warm on the coldest days without running constantly, the new one probably does not need to be bigger. A two-stage or modulating furnace runs at low output most of the winter, which reduces the downsides of a slightly large unit.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
In a mixed climate, an 80,000 BTU input furnace at 80% AFUE or a 70,000 BTU furnace at 96% AFUE. In a cold climate, about 100,000 BTU at 80% or 80,000 BTU at 96%.
What size furnace do I need for a 1,500 sq ft house?
Usually 60,000 BTU in warm and mixed climates and 60,000 to 80,000 BTU in cold climates, depending on efficiency.
How many BTU per square foot for a furnace?
About 22 to 46 BTU of output per square foot including a safety margin, from hot to very cold climates. In input terms at 80% AFUE that is about 27 to 57 BTU per square foot.
Is it better to have a bigger furnace?
No. A furnace that is too large short-cycles, heats unevenly and wears out sooner. Choose the smallest standard size that covers your heat loss with a modest margin.
Should my new furnace be the same size as my old one?
Not necessarily. Many older furnaces are oversized, and if you have added insulation or new windows since, your heat loss has dropped. A higher-efficiency furnace also delivers more output per BTU of input.
These results are estimates for planning. Final furnace size should come from a Manual J heat-loss calculation by a licensed contractor.