How to Size a Heat Pump for a Calgary Home
Size a heat pump for a Calgary home from a room-by-room CSA F280 heat loss, not from the size of the old furnace. Then check the heat pump's real output at low outdoor temperatures against that heat loss. Calgary's winter design temperature is about -30°C, and most heat pumps lose capacity as it gets colder, so the backup heat, electric or a gas furnace, is sized to cover the gap.
By the Ductscale design team. Reviewed by Raymond, founder and lead HVAC designer. Updated .
1. Start with the heat loss
The heat loss at the design temperature is the heat the house needs on a very cold day. The heat gain sets the cooling. Both come from a room-by-room CSA F280 calculation.
Sizing from the old furnace or from square footage usually oversizes. Many older furnaces were sized with a large margin, and copying that size carries the margin forward.
2. Check the output at cold temperatures, not the nameplate
The capacity printed on a heat pump's spec sheet is measured at a mild outdoor temperature. As it gets colder, output drops. Cold-climate models hold their output better, but they still lose some.
Use the manufacturer's extended performance data, which lists output at low outdoor temperatures, and compare it with the heat loss at those temperatures.
3. Find the balance point
The balance point is the outdoor temperature where the heat pump's output equals the house's heat loss. Above it, the heat pump carries the house on its own. Below it, backup heat makes up the difference. A lower balance point means the heat pump covers more of the winter.
4. Size the backup heat
Backup heat is either electric elements in the air handler or a gas furnace in a dual-fuel setup. It covers the gap below the balance point and keeps the house warm during defrost cycles.
Many owners size the backup to carry the whole house at the design temperature, so the house stays warm even if the heat pump stops. With dual fuel, the switchover temperature is set from comfort and energy cost.
5. Weigh the heating load against the cooling load
In Calgary, the heat loss is much larger than the heat gain. A heat pump big enough to carry the house at -30°C can be far larger than the cooling load, and a unit that is too big for the cooling load runs in short cycles in summer.
Variable-speed heat pumps can turn down, which gives more room to size for heating. We check both loads against the unit's data before recommending a size.
6. Check the ducts
Heat pumps deliver air at a lower temperature than a furnace, so they often need more airflow to deliver the same heat. Ducts sized for an old furnace may be too small, which means noise and less heat at the registers. A duct check, or a new duct design, makes sure the heat pump can deliver its rated output.
FAQ
Common questions
Can a heat pump heat a Calgary house on its own?
A cold-climate heat pump can carry a well-insulated house through much of the winter, but at -30°C most need backup heat. We size the backup so the house stays warm at the design temperature.
Should a heat pump be sized to the heating load or the cooling load?
It depends on the house and the unit. Variable-speed cold-climate units give more room to size for heating without poor summer performance. We check both loads and the unit's data before recommending a size.
Do I need new ducts for a heat pump?
Not always. We check whether the existing ducts can carry the airflow the heat pump needs, and size changes only where they cannot.