A winter service call: a Tosot heat pump immobilized by the cold
At AirGreen , we respond year-round to wall-mounted heat pump breakdowns caused by cold, snow, or simple electronic faults. This call, received in Châteauguay , on the South Shore of Montreal , concerned a recently installed Tosot heat pump , suspended on a wall bracket at the back of a single-family red brick home.
The customer contacted us after noticing that their heat pump was no longer blowing hot air. The indoor unit was working, but no heat was being felt, even after several restart attempts.
First on-site inspection
Upon arrival, we first noticed a clean and compliant installation , with a raised wall mount, which protects the unit well from snow accumulation on the ground—a positive point. However, a thin layer of ice had formed on the outdoor condenser fan, a sign that the defrost cycle was no longer functional.
By observing the compressor's behavior and the sounds emitted upon power-up, it was clear that the unit was not entering heating mode despite the thermostat command. We then undertook a complete diagnostic.
Detailed diagnosis
Our technicians followed a standard operating verification procedure:
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Power supply test : The voltage measured at the circuit breaker and the input terminal of the outdoor unit was stable at 240 V.
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Analysis of the control signal from the indoor unit: correct communication between the two boards.
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Defrost sensor check : The thermistor was showing abnormal resistance at -3°C, which was preventing the control board from activating defrost.
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Fan inspection : no mechanical obstruction, but a light layer of frost on the blades was slowing rotation.
The final diagnosis: defective temperature sensor . This small part, essential to the proper functioning of the heat pump, sends a false reading to the electronic board, blocking the switch to heating mode.
On-site repair
We replaced the outdoor coil temperature sensor with a genuine Tosot part, tested before installation. The new sensor displayed a value consistent with the actual ambient temperature.
We then restarted the system:
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The compressor started normally.
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The fan resumed smooth rotation.
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The defrost cycle automatically started after a few minutes, dissipating the ice that had accumulated on the coil.
Once defrosting was complete, the internal discharge temperature gradually rose to 34°C, confirming that the heat pump was returning to full efficiency.
Advice given to the client
Before leaving the premises, we explained to the customer some simple recommendations to preserve the performance of his Tosot device during the winter:
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Never turn off the heat pump during cold periods: the defrost cycle is managed automatically.
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Visually check the outdoor unit after a storm: remove snow from on top or around the unit.
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Schedule an annual maintenance before winter, to check the sensors and the correct calibration of the electronic system.
We also recalled that Tosot (a subsidiary of Gree) produces very reliable systems, but that, like any modern heat pump, they require preventive monitoring of their electronic sensors.
Final result
A few minutes after the repair, the Châteauguay home was comfortable again. Warm air was circulating again, the compressor was running quietly, and the customer could see normal power consumption on the control panel.
This intervention illustrates our approach: diagnose quickly, repair sustainably, and advise intelligently . At AirGreen, each service call is an opportunity to demonstrate our expertise and attention to detail — whether in Châteauguay , Brossard , Laval or Montreal .