Key takeaways
- Size to your home’s heat loss at design temperature, not to square footage or the old furnace’s size.
- Oversized units short-cycle; undersized units lean on backup heat. Both cost you.
- In Canada, the standard method is a CSA F280 heat-loss calculation — we recommend it.
- F280 sizes the heat pump, not your ducts — check that the ducts can carry the airflow, too.
Tons, BTUs and what they mean
A “ton” is a unit of capacity, not weight. Heat pumps are rated in tons for cooling, but what matters in Toronto is how much heat the unit can deliver when it’s cold outside.
Heat loss: the number everything depends on
Your home loses heat through walls, windows, the roof, the basement and air leaks. A heat-loss calculation adds all of that up, room by room, at your design temperature (around −18 °C to −20 °C in Toronto).
- Insulation levels in walls, attic and basement
- Window size, type and orientation
- Air leakage (a blower-door test is the gold standard)
- Ceiling heights and room layout
- Number of occupants and ventilation
If a quote sizes your heat pump from square footage alone, or simply matches your old furnace, ask for a heat-loss calculation. Old furnaces are often oversized.
The balance point
As outdoor temperatures drop, your home needs more heat while the heat pump can deliver a little less. Where the lines cross is the balance point. Below it, backup heat makes up the difference.
A good design puts the balance point low enough that backup heat runs only a small share of the winter — while not oversizing the unit for the rest of the year.
Oversized vs. undersized
| Oversized | Undersized | |
|---|---|---|
| What happens | Short, frequent cycles | Runs flat-out; backup heat kicks in often |
| Comfort | Temperature swings, clammy summers (poor dehumidification) | Cool rooms on the coldest days |
| Cost | Higher price, wasted energy | Higher bills from backup heat |
| Equipment life | More wear from cycling | Long run times — usually fine if designed for it |
When the heat load is bigger than your ducts
A CSA F280 calculation tells you how much heat your home needs. It doesn’t tell you whether your existing ducts can move enough air to deliver it — that has to be checked separately.
So if the report calls for 4 tons but your ducts were sized for the old 3-ton air conditioner, simply installing a 4-ton unit can cause:
- High static pressure in the ducts
- Too little airflow across the indoor coil
- Noisy vents and rushing air
- A blower that works harder and wears out sooner
- Less heat than the unit is rated for
- A frozen indoor coil in cooling season
There are four good ways to handle it:
It depends on your ducts, your budget and which rooms run cold. A good installer measures static pressure and airflow before recommending one — ask to see the numbers.
Questions to ask your installer
- Did you do a room-by-room heat-loss calculation? Can I see it?
- What design temperature did you use?
- What is the unit’s capacity at −15 °C and at the design temperature?
- Where is the balance point, and what backup heat covers below it?
- Are my ducts sized for the airflow this system needs?
- What static pressure did you measure on my existing ducts?
Common questions
Can I just match my old furnace’s size?
It’s a common shortcut but often wrong — many older furnaces are oversized. Size to the heat loss.
Should I size for heating or cooling?
In Toronto, heating usually drives the size. Variable-speed units handle the smaller summer load by running slower.
The report says 4 tons, but my old air conditioner was 3 tons. Is that a problem?
Only if your ducts can’t carry the extra air. Have the installer measure airflow and static pressure, then choose between upgrading the ducts, a unit that works at lower airflow, a smaller unit with backup heat, or adding ductless heads.