What size furnace do I need?
Heating load converted to the input rating a furnace is sold by, using its AFUE.
Furnaces are advertised by their input rating, and heat your house with their output. The difference is the efficiency, and ignoring it is the most common furnace sizing mistake there is.
Before you buy anything. This is a square-foot estimate, not an ACCA Manual J load calculation. A Manual J uses your building’s real construction, orientation and air leakage, and on a well-sealed house it usually comes out smaller than this. Use this to sanity-check a quote, not to replace one.
Input, output and AFUE
AFUE, the Annual Fuel Utilization Efficiency, is the fraction of fuel energy that becomes useful heat over a season. An 80% AFUE furnace with an 80,000 BTU input rating delivers 64,000 BTU of actual heat. A 96% AFUE furnace with the same input delivers 76,800.
So the sizing question is not "which furnace is 96,875 BTU", it is "which furnace outputs enough to cover the heat loss".
Worked example: 2,000 sq ft in climate zone 5
The same load at different efficiencies
| AFUE | Input needed | Furnace to buy | Its output |
|---|---|---|---|
| 80% | 96,875 | 100,000 | 80,000 |
| 90% | 86,111 | 100,000 | 90,000 |
| 96% | 80,729 | 100,000 | 96,000 |
A higher efficiency furnace needs a smaller input rating for the same heat, which occasionally drops it a whole size and saves money twice.
Standard furnace sizes
Residential gas furnaces are sold in these input ratings, so the calculated requirement is rounded up to the next one.
| Input BTU | Output at 80% | Output at 96% |
|---|---|---|
| 40,000 | 32,000 | 38,400 |
| 60,000 | 48,000 | 57,600 |
| 80,000 | 64,000 | 76,800 |
| 100,000 | 80,000 | 96,000 |
| 120,000 | 96,000 | 115,200 |