Why some wall-mounted heat pumps stop heating in extreme cold—and what you need to know
At AirGreen, we’re regularly called out across Montréal, Laval, Longueuil, the North Shore, and the South Shore to investigate wall-mounted heat pumps that stop functioning efficiently when temperatures drop below -20°C. While this issue is understandably frustrating for homeowners, it can usually be explained by a combination of technical factors that we’ll break down here. Based on what we’ve seen in the field, this article offers clear explanations along with reliable, actionable solutions.
Understanding the operating limits of wall-mounted heat pumps
Modern wall-mounted heat pumps are built to work in cold climates, but their effectiveness can vary greatly depending on the model and the technology it uses. At -20°C and lower, some units hit their operational threshold, drastically reducing their output or shutting off altogether.
Technical limits by model:
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Heat pumps with a standard compressor or without Inverter technology can struggle to extract enough heat from cold outdoor air.
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Lower-end units or systems that are improperly sized will shut down their heating cycle automatically to prevent thermal overload.
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Some heat pumps are simply not designed for deep Québec winters.
The refrigerant’s role in cold-weather performance
A wall-mounted heat pump’s ability to perform at very low temperatures depends heavily on the type of refrigerant it uses. Older units that run on R-410A typically lose effectiveness below -15°C. Newer systems that use R-32 or R-454B maintain much better performance at -25°C or even -30°C depending on the brand.
What we see at AirGreen:
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Gree Extreme and Fujitsu KZAH R-32 models provide consistent heating even at -25°C.
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On the other hand, older or entry-level units show a sharp drop in heating capacity as soon as temperatures hit -18°C.
Real-life case: service call in Blainville for a heat pump that stopped at -21°C
A client from the North Shore contacted us after noticing that their wall-mounted heat pump had stopped producing heat during a particularly cold night at -21°C. The unit was a 12000 BTU non-Inverter model, installed by a third party. After our diagnosis:
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The system had simply reached its maximum operating limit.
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There was no technical failure—just an automatic compressor shutdown for safety.
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We recommended replacing it with a Midea25 12000 BTU Inverter model, designed to maintain reliable heating down to -30°C.
Poor sizing: an aggravating factor in extreme cold
Even the best wall-mounted heat pumps will struggle if they aren’t properly sized for the space they’re heating. In cold weather, an undersized unit will run at full power without ever reaching the desired temperature.
Common mistakes we observe:
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Installing a 9000 BTU unit in a poorly insulated 600 sq. ft. living room.
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Overlooking heat loss from single-pane windows or high ceilings.
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Failing to consider room orientation (north-facing vs. south-facing) when choosing capacity.
Frost: the silent enemy that blocks heating
In extreme cold, the outdoor unit of a wall-mounted heat pump can accumulate frost, especially if it’s poorly positioned or not well maintained. This frost buildup prevents proper heat exchange and may cause the system to shut off or switch into frequent defrost cycles.
Our tips:
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Install the outdoor unit sheltered from prevailing winds, on a raised bracket.
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Schedule pre-winter HVAC maintenance, including a thorough cleaning of the unit.
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Choose models with an intelligent defrost mode, such as those from Fujitsu or Lennox.
How to keep warm even at -20°C
Choose a heat pump built for Québec’s climate:
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Inverter technology is essential.
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Refrigerant must be R-32 or R-454B for efficiency down to -30°C.
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Look for Energy Star certification and a high coefficient of performance (COP).
Add a backup heating system:
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Electric baseboards or an electric backup furnace.
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Smart thermostat to automate activation when your main unit’s performance drops.
Call AirGreen for a professional assessment:
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We assess your space, insulation, and usage.
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We recommend field-tested models, installed across the Greater Montréal area.
What our clients are saying
Testimonial from Longueuil:
“My heat pump kept shutting off at -18°C. After AirGreen stepped in, they installed a Gree Extreme 18000 BTU model. Even at -26°C, I’ve got heat. Super professional service.”
Testimonial from Laval:
“I realized my old unit just wasn’t built for our winters. AirGreen recommended a Fujitsu model with R-32, and it changed everything. No more plugging in space heaters.”
Why trust AirGreen with your cold-weather HVAC installations?
Our team installs hundreds of wall-mounted heat pumps every year in extreme weather conditions. We know which brands and models truly deliver in Québec. Our promise is simple: reliable comfort, even when temperatures plummet.
Whether you’re in Montréal, Laval, Longueuil, the North Shore or the South Shore, we’re here to help you choose a solution that’s reliable, long-lasting, and perfectly adapted to your home or business.