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Why Does Your Wall-Mounted Heat Pump Frequently Get Stuck in Defrost Mode? (And How to Permanently Fix the Issue)

A complete guide to understanding excessive defrost cycles on your wall-mounted heat pump and how to fix them effectively

At AirGreen, we often receive calls from clients in Montréal, Laval, Longueuil, the North Shore, and the South Shore who are concerned about their wall-mounted heat pump frequently switching into defrost mode, even when the weather is only moderately cold. While this function is normal to some extent, it becomes a problem when it occurs too often, affecting indoor comfort, energy performance, and the lifespan of your equipment.

What is defrost mode and why is it necessary?

When a wall-mounted heat pump operates in heating mode during cold weather, moisture in the air can condense and freeze on the outdoor unit. This frost buildup prevents the system from extracting heat efficiently.

The purpose of defrost mode:

  • Temporarily reverse the refrigerant cycle to melt the ice.

  • Restore proper heat exchange.

  • Protect the compressor and prolong equipment life.

However, if this mode activates too frequently or for too long, it can result in:

  • Noticeable drops in indoor temperature.

  • Higher energy consumption.

  • Uncomfortable heating fluctuations.

Case study in Montréal: a Sharp ZU1 24,000 BTU stuck in defrost

A residential client in the Rosemont–La Petite-Patrie area contacted us after noticing that his Sharp ZU1 24,000 BTU wall-mounted heat pump was going into defrost mode every 20 minutes, even at -5 °C.

Our diagnosis:

  • The outdoor unit was installed under a balcony, poorly ventilated, with little sun exposure.

  • Moisture was accumulating and freezing quickly.

  • The condensate tray had no slope, making drainage impossible.

Our solution:

  • Relocated the unit to a more open space.

  • Added risers to elevate the unit off the ground.

  • Insulated the refrigerant lines to prevent heat loss.

Result: Defrost cycles were reduced to once every 90 minutes, improving indoor comfort and reducing electrical consumption.

The main causes of excessive defrost cycles

1. Poor placement of the outdoor unit

  • Tight or poorly ventilated spaces

  • Near a gutter or sloped roof: water accumulation

  • No effective drainage

2. Faulty HVAC installation

  • Refrigerant lines too long or poorly insulated

  • No service access hatch for inspections

  • Improper leveling, which prevents water from draining properly

3. Incorrect settings

  • Thermostat poorly configured (setpoint temperature too high)

  • Faulty temperature or pressure sensors

4. Neglected HVAC maintenance

  • Dirty coils

  • Blocked outdoor fan

  • Frosted or miscalibrated sensors

Installation in Laval: Gree Extreme 18,000 BTU in a condo building

At a residential building in Chomedey, Laval, we were called in to review an HVAC installation done by another contractor. The Gree Extreme 18,000 BTU wall-mounted heat pump was switching to defrost mode 6 to 7 times per hour.

Findings:

  • The outdoor unit was installed too close to a wall, restricting airflow.

  • The refrigerant lines exceeded the manufacturer’s length limits.

  • No condensate drain was installed, causing ice buildup beneath the unit.

AirGreen intervention:

  • Reoriented the unit to improve airflow.

  • Shortened the refrigerant line length.

  • Installed a heated condensate tray.

Since our intervention, the unit now runs without excessive interruptions, and the client enjoys stable heating even at -20 °C.

How to prevent overly frequent defrost cycles

Choose an experienced installer like AirGreen

We always conduct:

  • A site analysis to select the ideal placement.

  • A check on refrigerant line distances.

  • Precise calibration of thermostats and sensors.

Schedule a complete annual HVAC maintenance

Including:

  • Coil cleaning

  • Function check on temperature sensors

  • Inspection of automatic defrost components

Choose a heat pump model designed for our climate

We recommend brands proven to perform well in cold climates, such as:

  • Gree Extreme

  • Sharp ZU1

  • Fujitsu KZAH

  • Midea Arctic

These systems are designed to operate efficiently down to -30 °C, with smartly managed defrost cycles.

Client testimonial in Longueuil: Fujitsu KZAH delivers guaranteed comfort

"Before, my heat pump kept shutting down constantly. Since AirGreen installed a Fujitsu KZAH 12,000 BTU, everything is stable. Even in extreme cold, the system doesn’t freeze up, and I get uninterrupted heating."
— Ms. Bernard, Longueuil

Practical tips for a worry-free winter

  • Never cover the outdoor unit, even in winter

  • Raise it at least 30 cm above the ground

  • Make sure nothing is blocking the air vents

  • Call AirGreen for an inspection if you hear unusual noises

Conclusion

At AirGreen, we know that defrost mode is a normal process, but it needs to be properly managed. A well-chosen location, correct HVAC installation, and regular maintenance are essential to avoid heating interruptions. With our hands-on experience across Montréal, Laval, Longueuil, the North Shore, and the South Shore, we help you get the most out of your wall-mounted heat pump, even in the harshest winter conditions.

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