An AC Freezing in the Heat Wave: A Frustrating Paradox
At AirGreen, every summer we respond to numerous service calls from homes and businesses across Montreal, Laval, Longueuil, the North Shore, and the South Shore for a problem that’s as unexpected as it is annoying: a wall-mounted or central air conditioner that freezes… just when it should be cooling. If your unit is covered in ice or is blowing warm air during a heatwave, chances are there’s a malfunction hiding behind the symptoms. Let’s break down the most frequent causes of this issue — and how our certified HVAC technicians handle it efficiently.
1. Poor Airflow: The #1 Culprit
Air must circulate freely around the evaporator coil to prevent cold air from accumulating. When air filters are clogged, ducts are blocked, or vents are obstructed or poorly placed, this can cause the coil to overcool, eventually freezing up.
What we see in the field:
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At a client’s home in Boisbriand, a single clogged filter that hadn’t been changed in months was blocking 60% of airflow.
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In a residence in Saint-Hubert, furniture placed in front of return vents caused the evaporator coil to freeze progressively.
Our solution:
A complete HVAC maintenance, including professional cleaning of filters, blower wheel, and ducts, followed by an airflow test and static pressure check.
2. Low Refrigerant Level or a Leak
A low refrigerant level (R-410A, R-32, or R-454B) can cause the evaporator coil to overcool. The result: ice buildup on the coil, reduced performance, and potential damage.
Real case in Laval:
A Fujitsu wall-mounted heat pump, 12,000 BTU, was freezing every other day. Our technicians found a micro leak in the refrigerant line connection, caused by a careless initial installation.
Our solution:
Pressure testing, nitrogen leak detection, repair of the faulty brazing, refill with the appropriate refrigerant, and final leak test.
3. Indoor or Outdoor Fan Failure
A faulty fan motor — whether in the indoor unit or the outdoor condenser — prevents hot or cold air from being expelled. This disrupts the system’s balance and can lead to evaporator coil freezing.
Case in Longueuil:
On a Zephyr 3 Ton central heat pump, the outdoor condenser fan was jammed by a branch. Without proper airflow, cold air was backing up inside the system.
What we do:
Full motor and capacitor diagnostics, replacement of defective parts, and system performance testing after repair.
4. Faulty Thermostat or Temperature Sensor
Wall thermostats or electronic sensors are crucial for regulating temperature. A miscalibrated sensor can keep the unit running when the desired temperature has already been reached, freezing the system unnecessarily.
Typical job in Montreal:
A Midea 18,000 BTU wall-mounted air conditioner was freezing repeatedly in an apartment on the Plateau. We discovered the indoor temperature sensor was poorly positioned — directly exposed to a constant stream of cold air.
Our fix:
Sensor repositioning, calibration, firmware update of the controller, and load testing.
5. Improper Installation or Incorrect Sizing
Choosing an oversized air conditioner for the space causes short cycles that don’t allow proper dehumidification. Excess humidity then condenses on the coils and freezes.
Frequent mistake seen on the South Shore:
New condos equipped with 24,000 BTU wall-mounted units in rooms smaller than 500 ft². Result: oversizing, irregular cycles, high humidity, and icing.
AirGreen recommendation:
A precise heat load calculation before any installation. We tailor our recommendations (BTU, brand, model) based on sun exposure, insulation, window surface, and actual room needs.
6. Neglected Maintenance: The Most Avoidable Cause
Regular HVAC maintenance can prevent 80% of freezing issues. That’s a fact. Yet many customers skip cleaning and yearly check-ups altogether.
Field observations:
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Dust buildup on the evaporator coil
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Mold growth or foul odors
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Refrigerant levels never checked
Our service:
At AirGreen, we offer preventive maintenance packages including full inspections, cleaning, fluid level checks, sensor calibration, and detailed reports.
Warning Signs That Your Air Conditioner May Be Freezing Up
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Air feels warm even though the unit is running
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Visible frost on pipes or evaporator coil
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Water pooling beneath the unit
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Unusual hissing sounds or straining motor noise
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Frequent restarts or unit shutting off unexpectedly
How AirGreen Resolves These Problems Quickly and Effectively
We treat every call about a freezing air conditioner as a high-priority issue. Our technicians respond quickly throughout Montreal, Laval, Longueuil, the North Shore, and the South Shore to:
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Pinpoint the exact cause of the freeze
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Immediately repair any accessible component
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Offer long-term solutions (replacement, upgrade, maintenance)
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Advise on best practices to prevent recurrence
Conclusion: Better to Prevent Than Deal with a Frozen AC in July
An air conditioner that freezes is more than just a nuisance. It’s often an early warning sign of a deeper issue that could damage your HVAC system. Thanks to our technical expertise, our knowledge of residential and commercial installations, and our field presence across Greater Montreal, we help you restore optimal comfort — with no stress and no surprises.