Advice
Why oversized equipment is bad equipment
Oversizing is the most common mistake in residential heating, and the hardest one to fix after the fact.
What an oversized unit does
It hits the setpoint too quickly, then shuts off. A few minutes later the temperature drifts and it starts again. This is short cycling.
The consequences are concrete:
Humidity never gets removed. In cooling, dehumidification takes run time. A unit that reaches 22 °C in eight minutes stops before it has dried the air. You get a cold clammy house — and the instinct is then to lower the thermostat further, which makes it worse.
Comfort is uneven. Blasts of very cold or very hot air, followed by long silences. Room-to-room temperature differences widen, because the air never has time to distribute.
Wear accelerates. Start-up is the hardest moment for a compressor. Multiplying start-ups shortens the life of the most expensive part in the system.
The bill goes up. Equipment that keeps restarting consumes more than equipment running long and slow.
Why the mistake is so common
Three shortcuts — all understandable, all wrong:
- “Match the old one.” If the old one was oversized, the error gets locked in for another twenty years.
- “Go a bit bigger to be safe.” Good instinct for a car engine, bad for an air conditioner, where run time is part of the function.
- “X BTU per square foot.” That rule of thumb ignores insulation, windows and orientation — which is to say, most of what determines the actual load.
What a load calculation does
It accounts for floor area, but also wall and attic insulation, glazing area and orientation, ceiling height, building airtightness and the number of occupants.
Which is why the evaluation happens in your house rather than over the phone.
The heat pump wrinkle
Sizing a heat pump is harder still, because one machine has to suit two different loads: cooling in summer and heating in winter. In Quebec, the heating load is usually the larger one.
Variable-capacity equipment is more forgiving — it modulates instead of running all-or-nothing — but that does not replace the calculation. It only reduces the cost of a small error.
In short
The right unit is the one that runs long and slow. It is counter-intuitive, and it is exactly what to aim for.
Read next
- Do heat pumps really work through a Quebec winter? Yes — and here is exactly what happens as the temperature drops, without the brochure vocabulary.
- My furnace keeps starting and stopping The possible causes, from the simplest — which you can check yourself in two minutes — to the one that needs a technician.
Frequently asked questions
How is the right capacity calculated?
With a heat-load calculation that accounts for floor area, but also insulation, glazing area and orientation, ceiling height, the number of occupants and how airtight the building is. Square footage alone is not enough.
My old unit was X BTU — can I just match it?
That is the most common way to carry an error forward. The previous unit may already have been oversized, and your house may have been insulated or re-windowed since. We redo the calculation.
Let’s talk about your project
One of our advisors comes to your home, measures what needs measuring and walks you through your options — free, and with no pressure.
Monday to Friday · 8:30 a.m.–12 p.m. and 1 p.m.–5 p.m. Outside business hours, our 24-hour emergency service is reachable at the same number.