Iced Heat Pump — Defrost Not Working

In heating mode, a heat pump extracts heat from outdoor air: its outdoor coil therefore runs colder than the surrounding air, and moisture deposits on it as frost. That is expected behaviour, and the unit clears it on its own through a periodic defrost cycle. The trouble starts when the ice stops going away: heat transfer collapses, backup heat takes over and the energy bill climbs — sometimes for weeks before anyone notices.

Why does the defrost cycle no longer clear the ice?

Defrost is a commanded sequence: the heat pump temporarily reverses its cycle to warm the outdoor coil, usually with the outdoor fan stopped during the operation. Three families of faults disrupt that sequence. Control: a failing defrost board or timer, or a temperature sensor that is misplaced, detached from the coil or out of calibration — the controller believes the coil is clear when it is not. Reversal: a four-way reversing valve that does not shift fully keeps the coil from warming up. Capacity: a low refrigerant charge, often the sign of a leak, or a fouled coil reduce heat exchange to the point where defrost ends without melting everything. On top of these comes a very Québec-specific constraint: defrost water that cannot drain and refreezes at the bottom of the unit because of blocked drainage, insufficient clearance or snow piled around the equipment.

What can you safely check?

Without opening the unit or touching the refrigerant circuit: clear snow and ice around the unit and free the air inlets and outlets, keeping a safe distance from the fins; watch a full sequence to note whether a defrost triggers at all and how long it lasts; record any code or indicator light shown at the controller to relay to the technician; and check how the backup heat behaves — running continuously confirms the loss of capacity. Never spray hot water on the coil, never chip at the ice and never force a fan that is frozen in place. Preventive maintenance scheduled before winter — coil cleaning, sensor and drainage checks — prevents most of these situations.

When should you call a technician?

As soon as the ice persists after a full clearing, the coil stays encased over several cycles, the outdoor fan is locked by ice, or backup heat runs non-stop. Checking a defrost sensor, a reversing valve or a refrigerant charge requires measurements and safe access — often on a roof. Montréal Combustion diagnoses and restores commercial heat pumps across Greater Montréal, the North Shore and the South Shore in a B2B setting: defrost sequence checks, refrigerant measurements, drainage and clearance corrections. 450-473-0909.

Frequently Asked Questions

Is some frost on the outdoor coil normal in winter?

Yes. In heating mode the outdoor coil runs colder than the surrounding air, so moisture deposits on it as frost: that is by design, and the defrost cycle clears it periodically. What is not normal is ice that survives from one cycle to the next, fully encases the coil, or builds into a block at the bottom of the unit.

Can we thaw a heat pump with hot water or a tool?

No. Water refreezes almost immediately and makes the build-up worse; a scraper or pick bends the fins and can puncture a tube, turning a defrost problem into a refrigerant leak. Stop the call for heat at the unit, switch to the building's backup heat source if there is one, and have a technician look at it.

Why does the ice always come back at the bottom of the unit?

The water produced during defrost has to drain away and leave the equipment. If the base pan drainage is blocked, the clearance under the unit is too small, or snow piles up around it, that water refreezes in place and the ice rebuilds from the bottom up with every cycle.