What happens to heat pump capacity below freezing
A common misunderstanding is that a heat pump either works or it does not. In practice it works across a wide range of outdoor temperatures, but how much heat it delivers and how efficiently it does so both change continuously as the air outside gets colder. Understanding that curve is the difference between a system that disappoints in January and one that performs as expected.
Capacity is rated at more than one temperature
Heating capacity is the amount of heat a unit can deliver, usually expressed in BTU per hour or kilowatts. Because that figure changes with outdoor conditions, reputable manufacturers publish it at several reference points rather than a single headline number.
For cold-climate models in North America, the most relevant published points are typically the rating near 8.3°C (47°F) and the rating near -15°C (5°F). The gap between those two numbers tells you how well a unit holds its output as conditions worsen. A model that retains most of its rated capacity at -15°C is far better suited to a Prairie or Northern Ontario winter than one whose output collapses.
How to read it: When comparing two units, look past the maximum capacity. Compare the capacity each one holds at the coldest published temperature, because that is the figure that has to cover your home on the worst day.
COP describes efficiency at a moment, not a season
The Coefficient of Performance is the ratio of heat delivered to electricity consumed at a specific outdoor temperature. A COP of 3 means three units of heat for every unit of electricity. As outdoor air cools, there is less ambient heat to move, so the COP of any air-source unit declines.
This is why a single efficiency figure can be misleading. A unit might post an impressive COP in mild shoulder-season weather and a much lower one in deep cold. Seasonal measures such as the Heating Seasonal Performance Factor average performance across an entire heating season and give a more honest picture for budgeting.
What variable-speed compressors change
Older single-speed units ran at full output or not at all. Modern cold-climate units use inverter-driven, variable-speed compressors that modulate output to match demand. In mild weather they run slowly and efficiently; in deep cold they ramp up to hold capacity. This modulation is a large part of why current cold-climate models maintain usable output at temperatures that would have stalled earlier generations.
The balance point ties it together
Every home loses heat at a rate that increases as it gets colder outside. A heat pump produces heat at a rate that decreases as it gets colder outside. The temperature where those two lines cross is the balance point. Above it, the heat pump alone keeps the home comfortable. Below it, something has to make up the shortfall.
Where the balance point falls depends on both the equipment and the building. A well-sealed, well-insulated home has a lower heat-loss line, so its balance point sits at a colder temperature and the heat pump carries more of the season on its own.
| Concept | What it tells you |
|---|---|
| Rated capacity at 8.3°C | Output in mild heating weather |
| Rated capacity at -15°C | Output you can count on in deep cold |
| COP at a given temperature | Efficiency at that exact moment |
| HSPF | Seasonal average efficiency for planning |
| Balance point | Temperature below which backup heat is needed |
Where to verify the numbers
Independent, publicly available references are the best starting point. Natural Resources Canada maintains guidance on heating with electricity and heat pumps, and the U.S. Department of Energy publishes general explanations of heat-pump operation that apply equally to Canadian conditions.
Continue reading
Sizing a cold-climate heat pump →
Backup heat and the defrost cycle →