The ratio, not the square footage
A stove is described to buyers in square feet, which is the one unit that carries no information. Two thousand square feet of 1978 ranch on an exposed ridge and two thousand square feet of a house built in 2019 differ by a factor of three in heat loss, and the stove does not know which one it is standing in. What a stove can be matched against is a load in BTU per hour, and that comes from the heat loss calculator or from an energy audit, not from a floor plan.
Working the numbers the form opens with: 32,000 BTU/hr of design loss over a 65 degree difference is a building UA of 492.3 BTU/hr per degree. At 35 degrees outside, which is where a lot of winters actually live, the same building only wants 17,231 BTU/hr. Against a 55,000 BTU/hr stove that is 31.3 percent of the rated output. The stove is being asked to spend the bulk of the winter at roughly a third of what it was built to do.
The turndown floor is where the trouble starts
Every stove has a lowest setting below which the fire is no longer burning properly. Call it 30 percent of maximum for the sake of arithmetic, which on this stove is 16,500 BTU/hr. Set that equal to the building load and solve for the outdoor temperature: 70 minus 16,500 divided by 492.3 gives 36.5 degrees. Above 36.5 degrees outside, the smallest fire this appliance can hold is already more heat than the house wants.
| Outdoor temperature | House wants | Share of the 55,000 BTU/hr stove |
|---|---|---|
| 5°F (design) | 32,000 BTU/hr | 58.2% |
| 20°F | 24,615 BTU/hr | 44.8% |
| 35°F | 17,231 BTU/hr | 31.3% |
| 36.5°F | 16,500 BTU/hr | 30.0% — the turndown floor |
| 45°F | 12,308 BTU/hr | 22.4% |
Nobody opens a window and burns on. What happens instead is that the air control gets closed past the point where the fire burns cleanly, because that is the only lever left. A choked fire runs the firebox cool, the wood keeps giving off gas that the cool firebox does not finish burning, and that gas goes up a flue that is also now cool. It condenses on the way. That is creosote, and creosote is the fuel for a chimney fire. Wet wood makes the same thing happen faster, which is why wet wood gets the blame, but a dry load in an oversized stove that is being strangled every evening produces it too.
Burn time cuts both ways
A 30 lb load at 5,800 BTU delivered per pound is 174,000 BTU. At the design output of 32,000 BTU/hr that is 5.44 hours, which is a real constraint if you want to sleep through a January night. At the 16,500 BTU/hr floor it is 10.55 hours, which sounds like the argument for a bigger stove, and it is the argument people use. The catch is that the long burn is the smouldering burn. A small stove reloaded twice a night and run with the air open is a cleaner flue than a big stove loaded once and shut down, and the small stove has the further advantage that on the two coldest nights of the year you can simply run it flat out.
Over a season the arithmetic is less dramatic. At 5,500 degree days the building takes 65.0 million BTU, which at 5,800 BTU/lb is 11,204 lb of wood, or 3.11 cords at 3,600 lb a cord. That is roughly 373 firebox loads, and the stove size barely moves the total, because the heat the building loses does not care what appliance replaced it. Size changes how the wood is burned, not how much.
Getting the two inputs that matter
The load comes from the heat loss calculator, and it is worth spending an evening on rather than guessing, because everything above scales with it directly. The BTU per pound comes from the firewood BTU calculator, which takes your species, your moisture reading and your appliance efficiency and gives you a delivered figure — the default of 5,800 here is a middling seasoned hardwood in a reasonably efficient appliance and will be wrong for green wood by a wide margin. If you buy wood rather than cut it, the firewood cord calculator tells you whether the stack that arrived is what you paid for.
The rated output is the input to distrust most. Read the specification carefully enough to see which figure it is: a peak output from a crib of kiln-dried softwood is a different number from an EPA test average, and manufacturers publish both. When a listing shows a wide output range, the low end is usually closer to what the appliance does on cordwood in a house.
What this page does not do
It does not say whether a stove may be installed in your space, whether your chimney will run it, or whether any clearance is met. Those are matters for the appliance listing, the code your jurisdiction has adopted, the building department, a certified chimney sweep and an installer. An existing chimney needs inspecting by a sweep before anything is connected to it, and no calculation replaces that. The hearth pad and clearance calculator lays out the footprint from the numbers in the listing, and the chimney draft calculator reports the physics of what the flue is doing, but neither of them issues a verdict either.
Questions people ask
Is a bigger stove not safer, since I can always run it low?
Running it low is the failure mode, not the escape hatch. A stove held below its clean-burning range keeps the firebox and the flue cool while the wood carries on giving off combustible gas, and that gas condenses in the chimney. On the default numbers here the stove reaches its lowest usable setting at 36.5 degrees outside, so every hour milder than that is an hour of choking the air. The stove that is comfortable to live with in a mild week is the one that can be run with the air open in a mild week.
The dealer sized it by square feet. Why does this page not?
Because square footage does not tell you the heat loss and the heat loss is the entire question. A 1,800 sq ft house with R-38 in the attic, new windows and a blower door result under 3 ACH50 might lose 22,000 BTU/hr at design; the same footprint uninsulated with leaky sash could lose 60,000. A square foot rating has to assume one of those. Run the heat loss calculator once and you have a number that describes your building rather than a category of buildings.
What number do I put in for the lowest usable output?
If the manufacturer publishes a minimum burn rate in pounds per hour, multiply it by your delivered BTU per pound and use that. If not, thirty percent of the rated maximum is a placeholder to see the shape of the answer, and you should treat it as such. The honest use of the field is to run it at 25 and again at 40 and look at how far the balance temperature moves; on the default inputs that spread is roughly 42.1 degrees outside down to 25.3.
Does an insert or a fireplace change the arithmetic?
The arithmetic is the same, but the two inputs change a lot. An open masonry fireplace delivers a very small and sometimes negative share of the wood energy to the room, because it is exhausting heated house air up the flue the entire time it burns. An insert with a properly connected liner behaves much more like a freestanding stove. Whichever it is, the delivered BTU per pound is the field that carries the difference, and the number for an open fireplace is far lower than the default on this page.
How much wood will I actually burn?
On the defaults, 3.11 cords over a 5,500 degree day season, but that figure inherits every assumption above it and the honest error bar is wide. The best estimate available to anyone is their own past winter: cords burned divided by the degree days that winter accumulated gives a rate per degree day for your building and your habits, and it beats this calculation outright. Use the degree day calculator on last winter and then trust your own number.