An underestimated issue that can seriously affect comfort and health in homes across Montréal, Laval, and the North Shore
In Québec, harsh winters and major temperature differences between indoors and outdoors often cause a sneaky problem: excess moisture caused or worsened by a wall-mounted heat pump that’s poorly used or improperly installed. This issue affects many homes in Montréal, Laval, Longueuil, and various cities on the South Shore and North Shore.
At AirGreen, we’ve observed that many of our clients don’t realize that their wall-mounted heat pump, although energy efficient, can actually make things worse when it’s poorly sized, poorly maintained, or used without proper ventilation.
Here’s everything you need to know to prevent indoor humidity issues, improve your air quality, and preserve your long-term comfort.
Understanding the link between wall-mounted heat pumps and indoor humidity
A wall-mounted heat pump works by transferring heat from outside to inside, or vice versa, depending on the season. But unlike a traditional furnace, it doesn’t burn fuel, which means it produces less dry air. In winter, this can lead to a buildup of moisture if the home isn’t properly ventilated.
In Boisbriand, we recently installed a Fujitsu KZAH 12,000 BTU R-32 in a single-family home. One week after installation, the client noticed condensation on the windows and an indoor humidity level of 70%. The system was working perfectly — but the home had no mechanical ventilation system.
1. Common mistake: installing a heat pump without evaluating the ventilation system
One of the most frequent errors is installing a wall-mounted heat pump in an older home without a proper mechanical ventilation system (HRV or ERV). Once the home is well insulated and the heat pump is running, the humid air has nowhere to go.
💡 AirGreen Tip: For every installation, we conduct a complete air renewal rate analysis and recommend adding an air exchanger if needed.
2. Wrong sizing = stagnant humidity
An oversized heat pump may heat the room quickly but then shuts off too soon, not allowing enough time for the system to effectively dehumidify the air. In Repentigny, a client had installed a 24,000 BTU unit in an 800 sq. ft. apartment. Result: poor temperature comfort and chronic humidity.
🔎 We replaced the unit with a Sharp ZU1 12,000 BTU, properly sized, and recommended sealing air leaks. Humidity dropped from 68% to 47% in just a few days.
3. Lack of maintenance = dirty filters, humid air
A heat pump that’s not maintained can’t properly dehumidify. Clogged filters reduce airflow and promote the growth of mold.
✔️ Our preventive maintenance includes:
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Full cleaning of filters and indoor coil.
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Inspection of the drain line to ensure proper condensation removal.
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Airflow testing using measuring instruments.
📍 In Sainte-Julie, we saved a Gree Extreme 18,000 BTU heat pump that was causing humidity simply because of a clogged drain.
4. Not using the dehumidification mode
Most modern wall-mounted heat pumps come with a “dry” mode, specifically designed to lower indoor humidity without overheating the room. Yet users often ignore this function.
🔧 In Verdun, a client complained about humid and cold air. The unit (MIDEA22 12,000 BTU 115V) was in cooling mode, even in the fall. After simply switching to dry mode, the humid feeling disappeared within 48 hours.
5. Forgetting the importance of natural ventilation
Even with a high-performance heat pump, it’s crucial to air out the home regularly. Bathrooms, kitchens, and basements naturally generate a lot of moisture.
🌬️ Our recommendations:
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Install timed exhaust fans in humid rooms.
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Open windows briefly each day.
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Add a high-efficiency HRV if the home is very airtight.
Case Study: AirGreen Installation in Laval
New semi-detached home, well insulated, with a main resident suffering from asthma. During the installation of a Gree Extreme 12,000 BTU wall-mounted heat pump, we:
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Integrated a Venmar AVS HRV with dual-speed settings.
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Programmed the heat pump in dry mode for seasonal transitions.
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Trained the client on how to adjust settings seasonally.
Result: improved air quality, regulated humidity, and optimal comfort, year-round.
Conclusion: Install smart, ventilate right
The wall-mounted heat pump is an excellent heating and cooling solution, but it must be part of an overall strategy for comfort and ventilation.
At AirGreen, we don’t just install — we assess, measure, adjust, and advise each client to prevent indoor humidity, protect air quality, and ensure lasting comfort, across Montréal, Laval, Longueuil, the North Shore, and the South Shore.