Heating & Cooling
Boiler Size Calculator
Estimate the heat output a hot-water or steam boiler needs for your home’s size and climate.
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Heating & Cooling
Furnace Size Calculator
Get a quick estimate of the furnace size your home needs. The result gives both the heat output required and the input rating to look for at your chosen efficiency.
What is a furnace size calculator?
Furnaces are sold by input rating — the fuel they burn per hour — but what heats your house is the output. This calculator estimates the output your home needs, then works back to the input rating at the efficiency you are considering.
How does it work?
Floor area is multiplied by a heat-load factor for your climate zone to get required output. Dividing by AFUE (as a decimal) gives the input rating, which is rounded up to a standard size. Hot-water heating? Use the Boiler Size Calculator. Replacing the air conditioner at the same time? Pair this with the AC Ton Calculator.
What information is needed?
- Heated floor area in square feet
- Your climate zone
- The AFUE rating of the furnace you are considering
How to use the Furnace Size Calculator
- Add up the heated floor area.
- Choose the climate zone for your region.
- Enter the AFUE of the model you are looking at.
- Select Calculate.
The formula
In plain terms, here is the calculation the tool runs:
Output = Area × Zone factor
Input = Output ÷ AFUE
- Zone factor
- 30–55 BTU/h per sq ft
- AFUE
- Annual fuel utilisation efficiency as a decimal
Worked example
Example
A 2,000 sq ft home in zone 4 with a 96% AFUE furnace.
- 2,000 × 45 = 90,000 BTU/h output
- 90,000 ÷ 0.96 = 93,750 BTU/h input
Look at a 100,000 BTU/h input furnace.
Frequently asked questions
Is a bigger furnace better?
No. An oversized furnace heats quickly, shuts off, and cycles often, which wears parts and leaves uneven temperatures.
What does AFUE mean?
Annual Fuel Utilisation Efficiency — the share of fuel energy that becomes heat in your home over a season.
Do I need a load calculation?
For a replacement, many contractors will do a Manual J calculation on request. It accounts for windows, insulation and air leakage room by room.
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