Defrost issues on wall-mounted heat pumps: causes, consequences, and urgent solutions you need to know
At AirGreen, every winter we receive an increasing number of emergency calls from homeowners in Montréal, Laval, Longueuil, the North Shore, and the South Shore facing a critical issue: their wall-mounted heat pump stops working due to a faulty defrost mode. This issue—often overlooked or misunderstood—can lead to total equipment failure if not quickly addressed. Here’s a deep dive into the problem, the mistakes to avoid, and urgent advice to preserve the lifespan of your HVAC system.
The vital role of defrost mode on a wall-mounted heat pump
The defrost mode is an essential automatic process on all wall-mounted heat pumps installed in Quebec, especially during extreme cold periods. When it’s -10°C and humidity is high, frost forms rapidly on the outdoor coils. Without proper defrosting:
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Frost builds up.
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Energy efficiency drops drastically.
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The system works harder, overheats, and may fail.
The automatic defrost cycle briefly reverses the heat pump operation to send hot refrigerant to the outdoor coil, melting the ice buildup. It’s a sensitive process, relying on multiple sensors and parameters.
Real case: misconfigured defrost in Laval on a Fujitsu KZAH
A client in Laval contacted us in early January: his Fujitsu KZAH 12,000 BTU wall-mounted heat pump, installed a year prior by another company, was no longer heating. After diagnosis:
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The defrost mode had been accidentally disabled during installation.
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The outdoor coils were covered in 8 cm of ice.
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The compressor had been running under strain for several days.
Our response:
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Reactivated the automatic defrost mode.
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Fully cleaned the coils.
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Tested all temperature sensors and the indoor thermostat.
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Installed a heat cable on the drain line, which was frozen.
Result: system brought back to full function with 100% restored efficiency, no equipment replacement needed.
Critical mistakes to absolutely avoid
These are the most common mistakes we see in the field:
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Manually disabling defrost mode using the remote or incorrect thermostat settings.
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Faulty or poorly positioned outdoor temperature sensor, misreading frost conditions.
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Frozen drain line, causing water backups into the unit.
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Outdoor unit installed too low, in direct contact with snow.
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Lack of preventive maintenance, leading to dirty coils and airflow restrictions.
How to check if your defrost mode is working properly
Here are the key signs that your heat pump’s defrost is working correctly:
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The outdoor unit blows cold air for a few minutes every 30 to 90 minutes: this is normal.
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You hear a click followed by a change in sound: the defrost cycle is starting.
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Water is draining out after the cycle: the frost has melted.
But if:
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The unit is persistently covered in frost or ice.
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Indoor air is no longer warm.
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The compressor runs non-stop.
…it’s time to call in a professional.
Successful installation in Longueuil: Gree Extreme 18,000 BTU heat pump
In November, a family in Longueuil reached out after two breakdowns caused by their old heat pump freezing over. We recommended a Gree Extreme 18,000 BTU, eligible for a $1,704 LogisVert rebate.
What we did:
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Fully removed the old system.
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Installed the new unit on an elevated bracket with adjusted slope.
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Properly configured the defrost mode via the indoor control panel.
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Final system verification to ensure everything ran automatically.
Since then, the family has gone through the winter without a single issue.
Our tips for safe and effective defrosting
1. Never manually disable the defrost cycle
Some users think they can improve performance by turning off defrost to "keep the heat going." This is a critical mistake.
2. Make sure water drains properly
The condensate drain line must be clear and ideally equipped with a heat cable to prevent freezing.
3. Schedule a maintenance check every fall
At AirGreen, our HVAC maintenance service includes:
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Detection of misconfigured settings.
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Inspection of faulty sensors.
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Coil condition check.
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Full defrost cycle diagnostics.
Why AirGreen is your best defense against frost damage
At AirGreen, we offer:
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Expertise in wall-mounted heat pumps built for Quebec winters.
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Over 1,000 successful installations in extreme conditions.
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Technicians trained to optimize every system setting.
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Custom solutions for all types of residential and commercial buildings.
Summary: defrost mode is a vital system that should never be overlooked
A misconfigured wall-mounted heat pump can be destroyed within days of severe cold. With proper setup, regular maintenance, and the right equipment, you can avoid all of these issues.
At AirGreen, we offer peace of mind, with high-performance installations in Montréal, Laval, Longueuil, North Shore, and South Shore, even during the harshest snowstorms.
Think your system is slow or not working right?
Contact AirGreen today for a defrost mode check-up or an optimized installation. Our experts are ready to respond quickly.