A harsh winter in Montréal puts all heating and air conditioning systems to the test, including multizone heat pumps. When an outdoor unit starts to freeze in the middle of winter, it can lead to stress, discomfort, and costly breakdowns if the problem isn’t addressed promptly. At AirGreen, we regularly receive calls from clients in Montréal, Laval, Longueuil, the North Shore and the South Shore facing this exact situation. This article provides a structured, clear, and concrete approach to identifying the causes of ice buildup on your unit, evaluating the risks, and applying expert-backed solutions recommended by our multizone HVAC specialists.
Understanding frost on a multizone heat pump: a normal phenomenon… up to a point
Light frost on an outdoor unit in winter is nothing unusual. When operating in heating mode, the heat pump extracts heat from the outdoor air – even when it’s very cold – and transfers it indoors. During this process, ambient humidity may condense and freeze temporarily on the outdoor coils. That’s why modern multizone systems are equipped with an automatic defrost cycle.
However, when the frost becomes excessive or persistent, it can block airflow, cause a drastic drop in performance, damage the fan, or even lead to a complete system failure. At this stage, professional intervention is required.
Common causes of excessive frost on a multizone heat pump in Montréal
1. Defective automatic defrost cycle
Every multizone heat pump, whether it’s made by Fujitsu, Gree, Mitsubishi, or another brand, uses a reverse-cycle defrost system. If this mechanism fails (faulty sensors, control board errors, failure to reverse cycle), the unit continues to accumulate frost.
2. Blocked or poorly directed airflow
Snow, ice, or debris buildup around the outdoor unit restricts airflow and worsens frost formation. A poor installation orientation can also expose the heat pump to harsh prevailing winds.
3. Lack of seasonal maintenance
Skipping preventive maintenance is a major contributor. Dirty coils, a clogged fan, or soiled sensors may prevent the system from detecting frost or properly triggering the defrost cycle.
4. Low refrigerant or leak
A refrigerant leak (R-32 or R-410A) causes a drop in system pressure, reducing efficiency and accelerating frost accumulation.
What we do at AirGreen to diagnose and resolve the problem
When we’re called in to service a frozen multizone heat pump in Montréal, our approach is methodical and focused on fast, long-lasting solutions.
Full inspection of the outdoor unit
Our technicians assess the unit to evaluate frost severity, and check the fan, sensors, and coils. We also verify the defrost cycle performance by simulating operations.
Refrigerant level check
We test system pressure to detect any leaks or undercharge. If needed, we repair the lines and refill with the manufacturer-specified refrigerant.
Installation condition analysis
We check whether the unit’s placement meets clearance recommendations and ensure airflow isn’t obstructed. If needed, we reposition or elevate the unit to avoid snow buildup.
Full or corrective maintenance
We perform a deep cleaning of the outdoor unit, including coils, fan, sensors, casing, and drain lines. This ensures optimal performance even at -25°C.
Case study: Successful intervention in Ahuntsic (Montréal)
A client contacted us last January regarding a 36,000 BTU Gree Multi+ multizone heat pump, installed two years prior. The outdoor unit was completely frozen, preventing any heating. Upon inspection, we identified a faulty outdoor temperature sensor responsible for triggering the defrost cycle.
We replaced the sensor, cleaned the unit, added supports to elevate it, and installed a perforated aluminum snow guard to prevent accumulation. The result: the heat pump now runs flawlessly, even at -28°C.
How to prevent your multizone heat pump from freezing again?
Schedule annual maintenance with AirGreen
We recommend a full preventive maintenance service before winter, including cleaning, calibration, and testing of critical components.
Regularly inspect your unit
During the winter, make sure that frost dissipates periodically (approximately every 30 to 90 minutes). If that’s not the case, contact us immediately.
Keep the unit clear
After each snowstorm, clear the base and sides of the outdoor unit to ensure proper airflow.
Why choose AirGreen for your multizone heat pump maintenance and repair?
We specialize in installation and servicing of multizone heat pumps in Montréal, Laval, Longueuil, the North Shore and the South Shore. Our technicians are trained to service the most recognized brands on the market: Fujitsu, Gree, Haxxair, Zephyr, Mitsubishi Electric, Daikin, Tosot, and more.
Our expertise is built on:
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Rapid intervention during winter emergencies.
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Accurate diagnostics using specialized tools.
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Full transparency on repairs and costs.
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Practical recommendations to prevent future breakdowns.
Key practical tips for multizone heat pump owners
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Never attempt to chip off ice with a sharp object or pour hot water on the unit.
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Don’t shut off power during frost, unless the unit is out of use for an extended time.
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When in doubt, rely on HVAC specialists.
AirGreen, your trusted winter partner
In Montréal, winter spares no one. But with the right habits, the right installation, and rigorous maintenance, your multizone heat pump can provide unparalleled comfort, even during the harshest cold spells.
At AirGreen, we believe every home deserves a reliable, long-lasting, and energy-efficient heating system. Contact us for an inspection or rapid service at the first signs of freezing.