Guides · 2026-08-14
What happens to a heat pump in a Montreal cold snap
The most common winter call is some version of: it was fine in November, and now it does not seem to keep up. Usually nothing is broken.
A heat pump extracts heat from outdoor air. The colder that air, the less heat there is to extract and the harder the machine works to move it.
Capacity falls exactly when you need it most
Heat loss from your home rises as the outdoor temperature drops. The heat pump's output falls at the same time. Those two lines cross at the balance point — the outdoor temperature below which the unit alone can no longer hold the setpoint.
Below that, something else has to contribute. In a typical Quebec installation that something is the electric baseboards you kept.
Defrost is normal and it costs energy
In cold, damp conditions frost forms on the outdoor coil, and the unit periodically reverses to melt it. During defrost it is not heating your home, and it is consuming electricity to clear ice.
Frequent defrost cycles in freezing drizzle are expected behaviour. A unit encased in solid ice, or one defrosting constantly in dry cold, is not.
Telling normal from a fault
Normal: longer run times in deep cold, occasional defrost cycles, baseboards engaging on the coldest nights, reduced output at −25 °C.
Worth investigating: ice building faster than defrost clears it, airflow noticeably weaker than last season, a musty smell, or the unit failing to keep up in merely moderate cold that it handled last year. That last one often points at fouling rather than the weather.
What this does to the savings estimate
Every hour the backup carries the load is an hour at a COP of 1.0. This is precisely why a seasonal average COP is lower than the rating on the box, and why our calculator asks for a seasonal figure rather than a peak one.
A colder-than-average winter shifts the real saving down. A range, rather than a single number, is the honest way to express that.
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Heat pump vs baseboard calculator · Symptom diagnostic · Heat pump cleaning Montreal · Heat pump not heating: what to check
FAQ
Should I turn the heat pump off in extreme cold?
Generally no — follow the manufacturer's guidance for your model. Modern cold-climate units are designed to keep operating and to hand over to backup as needed.
Is ice on the outdoor unit a problem?
Some frost is normal and defrost handles it. A unit substantially encased in ice, or one where ice returns immediately after a defrost cycle, is worth looking at.
My unit worked last winter and struggles this one. Why?
If the weather is comparable, the change is more likely in the equipment than the climate. Accumulated fouling on the coil and blower is the common cause, and it develops gradually enough to go unnoticed.