Heat pump in extreme cold: why it falls behind when the mercury drops
Updated
The heat pump handled the whole fall without a hiccup. Then the first real January cold snap arrives: the house is barely warm, the outdoor unit is caked in ice, or the display shows an error code. Nothing is necessarily broken — but at an empty cottage, a heat pump that falls behind with nothing backing it up is a pipe at risk of freezing.
What to do when it can’t keep up
1. Make sure backup heat takes over
Most Québec installations keep a second system: electric baseboards, an electric furnace, a propane or oil appliance. Turn that system’s thermostat up and make sure it starts. If nothing is heating, that’s the priority — before the heat pump itself.
2. Read the display or indicator light
Write down the error code or blink pattern and take a photo. The manufacturer’s manual explains what it means. Don’t keep resetting it: a unit that keeps going back into error has a reason.
3. Look at the outdoor unit — from a distance
Is it buried under snow that slid off the roof, locked in ice, blocked by a snowbank? If snow is blocking the air intake, clear it gently by hand or with a broom, never with a shovel or pick against the fins. If thick ice encases the unit, don’t try to break it: call a technician.
4. Let the defrost cycle do its job
In cold, damp weather a heat pump regularly goes into defrost: the fan stops, steam may rise from the unit, and the air coming out inside turns lukewarm. That’s normal. What isn’t normal is ice that keeps building and never melts.
5. Check the breaker — don’t keep resetting it
Through the panel door, see whether the heat pump or backup heat breaker has tripped. A breaker that trips again after being reset once needs an electrician.
6. Call a qualified contractor
For a persistent error code, a unit locked in ice or a system that won’t restart, call the contractor who installed it or another RBQ-licensed refrigeration and heating contractor.
Why it falls behind in deep cold
It pulls heat from outside. A heat pump doesn’t make heat: it draws it from the outdoor air and moves it inside. The colder it is outside, the less heat there is to draw, and the harder the unit has to work to deliver the same amount. Its output drops exactly when the house needs the most.
Every model has a limit. The manufacturer lists a minimum outdoor operating temperature in its specifications. Cold-climate models run lower than standard ones, but they all reach a point where they can’t keep up or shut down to protect themselves. Natural Resources Canada publishes information on heat pumps, including cold-climate models and how to size backup heat.
Frost and ice. The outdoor coil gets colder than the surrounding air, and moisture freezes on it. The defrost cycle melts it, but that water needs somewhere to drain. If it refreezes under the unit, ice builds cycle after cycle until it jams the fan or the fins.
Townships realities. Mountain cottages in Sutton or up in Potton get heavy snow and wind. A unit set too close to the ground, under an unprotected roof edge or in a corner where snow drifts ends up buried after a big dump. And cold snaps often come with peak demand and local outages.
Backup heat gets forgotten. Some owners shut off the baseboards once the heat pump is in, or set the backup thermostat so low it never kicks in on time. The day the heat pump falls behind, nothing picks up the slack.
Prevention: what you can do, and what belongs to a technician
What an owner can do:
- Keep backup heat working and set so it starts if the heat pump can’t keep up, following your installer’s advice.
- Clear snow off the outdoor unit after a storm, with no tools against the fins.
- Follow Environment Canada’s extreme-cold warnings and plan a visit after the cold snap.
- Write down the model and its minimum operating temperature as listed by the manufacturer.
- Leave a stand-alone thermometer in the coldest room.
What belongs to an RBQ-licensed contractor:
- Annual heat pump servicing and a check of the defrost cycle.
- Raising the outdoor unit on a stand or adding a suitable cover.
- Fixing drainage that freezes under the unit.
- Adding or repairing backup heat and its circuits (master electrician).
In an empty house, how long before anyone notices?
A heat pump that falls behind on the coldest night doesn’t warn anyone. If backup heat starts, the house holds. If it doesn’t, rooms cool down starting with the basement and exterior walls, and the first visible sign is often a frozen pipe. Cold snaps can last several days: a visit during or just after one is the difference between a logged error code and water damage at the thaw.
How often someone stops by sets the maximum length of this kind of problem.
What we look at on a visit
Every visit, we read the main thermostat and compare it with the agreed setting, then read the thermometer in the coldest room to see whether the gap is widening. We check that the heating equipment is running: heat pump starting, baseboards warm, error code showing or not. We confirm there’s power and look through the panel door for a tripped heating breaker, never resetting anything without your go-ahead.
If the heat isn’t holding, we call you the same day with photos, and bring in your installer or an RBQ-licensed contractor if needed.