How to size a furnace for your house

What does a furnace size calculator actually do?

Most online tools take a few inputs, such as floor area, climate zone, insulation level and number of storeys, then multiply the area by a factor and return a BTU-per-hour number. That is a shortcut. It guesses at heat loss from averages, and a house with new windows and a tight envelope will differ from a neighbour's draughty one of the same size.

The result is still useful as a sanity check. If a contractor proposes a furnace far above the calculator's range, ask why. Treat the calculator output as a range to question, not an order to buy. The figure that should guide a purchase is the heat loss of the actual house, as load calculations are explained in a separate guide.

How is furnace size measured?

Furnace size is heating capacity in BTU per hour, shown on the data plate. Two numbers appear: input, the energy in the fuel burned each hour, and output, the heat delivered to the house after losses. Output equals input multiplied by efficiency. A furnace with a 100,000 BTU/h input and a 96 percent efficiency rating delivers about 96,000 BTU/h. Compare the output with your heat loss, and use the same unit throughout. In metric terms, 1 kilowatt is about 3,412 BTU/h.

The size is not the physical size of the cabinet. Two furnaces with the same cabinet can have different capacities, and a high-efficiency model may be smaller than an old one of equal output. The efficiency side is described in the guide to AFUE ratings.

How does a heat loss calculation work?

A heat loss calculation adds up how much heat the building envelope loses at the design temperature, which is a cold outdoor temperature set for your location. For each part of the envelope, the loss is its area, times its conductance, times the temperature difference between inside and outside. Air leakage and ventilation are added on top.

Here is a small example with stated assumptions, for one component only. Take 100 square metres of wall with an insulation value of RSI 3.5, which is a conductance of about 0.29 watts per square metre per kelvin. With 20 degrees inside and minus 20 outside, the difference is 40 degrees. The wall loses about 100 x 0.29 x 40, roughly 1,150 watts, or about 3,900 BTU/h. Windows, doors, the roof, the foundation and air leakage each add their own figure, and the total is the heat loss of the house.

In Canada, qualified contractors commonly follow a residential load calculation standard, and the calculation should use your local design temperature and measured or estimated air tightness. A blower door test improves the air leakage input.

Why is a bigger furnace not a better furnace?

An oversized furnace heats the house too quickly and shuts off, then starts again soon after. These short cycles cause temperature swings, extra noise and wear, and the furnace spends little time at its efficient steady operation. Condensing models are especially affected, since they gain efficiency from steady running. A furnace sized slightly above the heat loss is normal, to cover extreme days, but one that is far larger wastes money, both in purchase price and in running cost.

Houses change. If you added insulation, replaced windows or sealed air leaks since the old furnace went in, its capacity may now be too large. That is a reason to run the calculation again rather than copy the old label, as the new furnace size and efficiency guide explains. Two-stage and modulating models offer more forgiveness, covered in the two-stage and variable-speed comparison.

What should you ask the contractor?

Ask for the heat loss number and the assumptions behind it: design temperature, insulation values, window types and air leakage. Ask what capacity they propose, and why. A contractor who sizes by square footage or by copying the old furnace is guessing. Upgrading the envelope first can also reduce the size you need, and an energy audit shows where the biggest gains are. The furnace replacement guide places sizing in the whole decision.

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