Furnaces are sized in BTU per hour, air conditioners in tons. Get the number right and the house heats evenly and quietly, and the equipment lasts. Get it wrong, usually too big, and the system short-cycles, rooms swing between hot and cold, and parts wear out early. This guide explains how sizing works for a house around Hamilton, and how to tell whether a contractor is doing it properly.
The units
A BTU, or British thermal unit, is a small amount of heat. A furnace is labelled by its input, the gas it burns per hour, and its output, the heat it delivers. A 60,000 BTU/h input furnace at 96% efficiency delivers about 57,600 BTU/h of heat. Air conditioners and heat pumps are sized in tons, where one ton moves 12,000 BTU/h, so a 3-ton AC removes about 36,000 BTU/h.
What the right size means
The right size matches the heat the house loses on a cold design day. In Hamilton that day is about −18 °C. The furnace should be able to keep the house at 21 °C at that temperature, with a modest margin. On every warmer day, which is nearly all of them, it has spare capacity. That is fine for a two-stage or modulating furnace, which can run at low output; it is why oversized single-stage furnaces cycle on and off so often.
Why the old furnace is a poor guide
Many furnaces installed in the 1970s to 1990s were sized with generous rules of thumb, sometimes doubled for safety. Since then, the house may have had new windows, attic insulation, a new front door or air sealing, all of which cut heat loss. A furnace the same size as the old one is often far too big. It is common for a properly calculated replacement to come in at 20 to 40% smaller.
Rules of thumb, and their limits
Rough heating figures for southern Ontario homes run from about 18 BTU/h per square foot for a new, well-insulated house to 40 or more for an older, draftier one, counting heated basements. For cooling, a typical house needs one ton per 500 to 750 square feet of above-grade floor area. Our furnace and AC size calculator uses those ranges to give you a bracket.
A bracket is useful for spotting a quote that is wildly off, but it is not a design. Two houses of the same size can differ by half in heat loss depending on windows, insulation, air leakage, the basement and which way they face.
The proper method: CSA F280
CSA F280 is the Canadian standard for calculating a house’s heat loss and heat gain. The Ontario Building Code requires it for heating and cooling systems in new houses, and HRAI-trained designers use it for replacements too. A proper calculation works room by room:
- Wall, ceiling and floor areas, and their insulation levels.
- Window and door areas, types and directions.
- Estimated air leakage, or a measured figure from a blower door test.
- The basement: how deep, whether it is insulated and how it is used.
- Local design temperatures for winter and summer.
The result is a total heat loss and a heat gain, and room-by-room numbers that show whether the existing ducts can deliver the right amount of air to each room. It takes a contractor an hour or two with good software. Ask for it, and ask to see the summary.
What oversizing costs you
- Short cycling: frequent starts wear igniters, inducers and controls faster.
- Uneven rooms: the thermostat is satisfied before air reaches the far rooms.
- Noise: a big furnace on small ducts moves air too fast and whistles at the registers.
- Clammy summers: an oversized AC cools the air quickly and shuts off before removing humidity.
- Higher price: you pay more for capacity you do not use.
And undersizing
Undersizing is rarer but real, usually after an addition or a finished basement was added onto the old system. The furnace runs constantly on the coldest days and cannot hold the temperature. A heat-loss calculation catches it. For heat pumps, the design is often deliberately sized to cool and handle most of the heating, with backup heat for the coldest hours.
Additions, finished basements and heat pumps
Sizing gets harder when the house has changed. An addition or a finished basement adds heat loss the original furnace never accounted for, and the existing ducts may not reach the new space well. A room-by-room calculation shows whether the new space needs its own ducts, a separate zone or a ductless head instead of a bigger furnace. Heat pumps add another wrinkle: in a hybrid system, the heat pump is usually sized to the cooling load and to cover most heating hours, with the furnace sized for the full heat loss. An all-electric heat pump is sized closer to the heat loss, with backup heat for the design day. The right choice depends on the house and on how much backup heat you want to pay for.
Ducts matter as much as the furnace
Many older houses in the lower city and on the Mountain have undersized return ducts, so even a correctly sized furnace cannot move enough air. A good contractor checks the static pressure in the ducts and may recommend an extra return or larger trunk. That small cost often fixes comfort problems that a bigger furnace would only make worse.
Getting it sized properly
Send one request and ask the contractor for a CSA F280 heat-loss and heat-gain calculation as part of the quote. Read furnace installation and central air installation for what else to look for.
Questions people ask
What size furnace do I need for a 2,000 square foot house?
Often between 60,000 and 80,000 BTU/h input in the Hamilton area, but it can be less in a well-insulated newer home. A heat-loss calculation gives the real figure.
How many BTU per square foot for a furnace in Ontario?
Roughly 18 to 40 BTU/h of heat loss per square foot, depending on the age and insulation of the house. That range is too wide to size from, which is why a calculation matters.
What size air conditioner do I need?
About one ton per 500 to 750 square feet of above-grade floor area in southern Ontario, adjusted for windows, sun and insulation.
Is a bigger furnace better?
No. An oversized furnace short-cycles, makes rooms uneven and wears out faster. The right size is the one that matches the house’s heat loss on a design day.