My heat pump stops heating when it's cold: what to check before you call

In short: in cold weather, a heat pump that stops blowing warm air for a few minutes, gives off steam or makes a "whoosh" is often just defrosting, and that's normal. Before you call, check the mode (Heating), the filter and the space around the outdoor unit. If it still won't heat, if the ice never melts or if an error code shows up, call a technician.

The first cold mornings are here, and with them the same questions: "My heat pump is blowing cold air, is it broken?", "There's smoke coming out of the unit outside!", "My baseboards keep coming on!" Good news: most of the time, that's just how a heat pump works in winter. Here's how to tell the difference, according to Hydro-Québec, Natural Resources Canada (NRCan) and our technicians.

What's normal in winter

Defrosting: no warm air, steam and noises

When it's around 0°C or colder, moisture in the air freezes on the outdoor unit's coil. That frost cuts its efficiency, NRCan explains, so the heat pump regularly switches into defrost mode: it reverses its cycle to send heat to the outdoor unit and melt the frost, and the outdoor fan stops.

For those few minutes, it's normal:

• for the indoor unit to stop blowing, or to blow cooler air;
• to see steam rising from the outdoor unit (it isn't smoke);
• to hear a "whoosh" or a click as the cycle reverses, then water draining under the unit.

According to NRCan, your baseboards or other backup heat may also kick in during defrost. Then the heat pump goes back to heating on its own.

Air that feels lukewarm

A heat pump heats a larger volume of air to a lower temperature than a furnace: NRCan mentions about 25 to 45°C, compared with 55 to 60°C for a furnace. So the air can feel lukewarm on your hand even while the house is warming up. We cover this in our article How to set your heat pump for winter.

Baseboards coming on during cold snaps

The colder it gets outside, the less heat an air-source heat pump can draw from the outdoor air. According to NRCan, most models also have a minimum operating temperature, often between -15°C and -25°C for newer units; below that, your backup heat does the job. On the coldest days, it's normal for the baseboards to take over: that's what they're there for. Hydro-Québec points out, though, that southern and central Québec average fewer than 15 days below -20°C per winter.

Check these before you call

1. Mode and temperature

Make sure the heat pump is in Heating mode, not Auto, Cooling or Dry: Hydro-Québec recommends Heating mode all winter. Also check that the set temperature is higher than the room temperature.

2. Baseboards set lower than the heat pump

If your baseboards are set as high as the heat pump, they heat first and the heat pump seems to "do nothing." Hydro-Québec and NRCan recommend setting the baseboards 2 to 3°C lower than the heat pump.

3. The filter

A dirty filter cuts airflow. According to Hydro-Québec, it can reduce your heating system's efficiency by 10 to 30%. Clean it as your unit's manual says; see also our article How to clean your heat pump filters.

4. Snow around the outdoor unit

Hydro-Québec recommends keeping the outdoor unit clear so it doesn't get blocked. After a storm, or if snow slides off the roof onto it, gently clear the loose snow around the unit, by hand or with a soft-bristle broom.

Don't: don't try to remove ice yourself. Don't chip, scrape, pry or pour hot water on the unit, don't open its panels, don't touch the wiring and don't cover the unit with a tarp or cover. The fins and lines are easily damaged.

A bit of frost that comes and goes is normal: the defrost cycle takes care of it. A unit stuck in ice is a job for us.

5. The breaker and error code

If nothing lights up, check whether the heat pump's breaker has tripped in your electrical panel. If it trips again, don't keep resetting it: call a technician. If a light is blinking or an error code appears, write it down (or take a photo): it helps the technician a lot.

When to call a technician

Call us if, after these checks:

• the heat pump blows cold air for long periods, outside of defrost cycles;
• ice keeps building up on the outdoor unit and never melts;
• you hear unusual noises (grinding, metallic banging, heavy vibration);
• an error code keeps coming back or the breaker trips;
• water is leaking from the indoor unit.

NRCan recommends having a qualified contractor service your heat pump every year. A tune-up before winter often catches a problem before the first deep freeze. See also our maintenance tips (in French).

Sources

• Hydro-Québec, How to use your heat pump in winter to maximize savings
• Hydro-Québec, Seven tips for heat pumps
• Natural Resources Canada, Heating and Cooling With a Heat Pump (Defrost cycle, Supplementary heat sources, Operation and Maintenance sections).