Why Does My Heat Pump Freeze Up in Winter?

Frost is the design working, not failing

An air-source heat pump collects heat from outdoor air. To do that, its outdoor coil — the evaporator in heating mode — has to be colder than the air it is collecting from, typically by a noticeable margin, because heat needs a temperature difference to flow across.

So two conditions have to be met at once, and in heating weather they usually are: the coil is below the air's dew point, so moisture condenses out of the air onto it, and the coil is below freezing, so that moisture arrives as ice instead of draining away as water.

This is why frost appears far more aggressively in mild, damp weather than in a deep freeze. Air a few degrees above freezing holds a great deal of water vapour; air well below freezing holds very little. The worst frosting conditions are a damp day near the freezing mark, not the coldest night of the year.

Frost matters because it insulates the coil and blocks the gaps between the fins, choking the airflow. Both effects cut the heat the machine can collect, so left alone, frost makes itself worse.

How the machine removes it

The trick a heat pump uses is elegant: it runs itself backwards for a few minutes.

A reversing valve sends hot compressor discharge gas to the outdoor coil instead of indoors. The coil becomes the hot surface, melts the ice from within, and the meltwater drains away — producing the cloud of steam that makes people think something has caught fire. The outdoor fan normally stops so the heat stays in the coil, and because the indoor side is now the cold side, many systems bring in a backup heater to prevent the house cooling during the cycle.

What triggers it matters. Older systems ran defrost on a timer whether or not frost had formed, spending energy on cycles nobody needed. Modern systems use demand defrost: sensors watch coil temperature, air temperature and the pressure difference a blocked coil produces, and call a cycle when the evidence says there is ice.

Each cycle takes heat back out of the building. Defrosting occasionally is normal; defrosting constantly means a large fraction of the machine's output is going into un-heating the house, which is a symptom rather than a quirk.

Telling normal frost from a fault

Here is the distinction that matters most, because almost every genuine problem announces itself in the same few ways.

Two patterns deserve particular attention. Ice in the base pan means meltwater has nowhere to go: the drain is blocked, the unit sits too close to the ground, or debris has filled the space beneath it. Each defrost cycle then adds to the ice instead of removing it. And frost that rebuilds almost immediately says the coil is running colder than design, which points either at airflow — a dirty coil, a failing fan, snow drifted against the unit, leaves in the fins — or at a low refrigerant charge, which is a technician's job.

What you can do, and where to stop

The safe interventions are all about giving the machine the conditions it was designed for.

Clear snow and leaves from the coil and the fan, and keep vegetation back from the fins. Make sure water from a roof or gutter is not landing on the unit, where it freezes in place and defeats every defrost cycle. Check that the unit stands clear of the ground and that the space beneath it drains, and keep the coil clean — a fouled coil runs colder for the same job.

What not to do: never chip ice off, because the fins and the tubing behind them puncture easily and a refrigerant leak is a far larger problem than the ice was. Do not switch off the defrost function. Do not block the unit to "keep it warm". And do not keep resetting a system that is defrosting continuously — it is reporting something, and continuing to run it in that state achieves nothing.

The ground-loop version of the same problem

A ground-source machine has no outdoor coil in the air, so it cannot frost in this way at all. It can still freeze, on the other side — and the cause is entirely different.

A closed loop extracts heat from the ground, and the ground replaces it slowly by conduction from the surrounding mass and from seasonal recovery in summer. If the loop is short for the load, extraction outruns replacement, and the fluid returns colder each year — the drift described in how geothermal heat pumps work. Should the entering fluid approach the freezing point of the antifreeze mixture, ice can form inside the heat exchanger, which restricts flow and can damage it.

Low flow does the same thing locally: a partly blocked strainer, trapped air or a failing circulator means each litre of fluid gives up more heat than intended, so it leaves colder even though the loop as a whole is fine. Open-loop systems have their own version, where the well water itself is the constraint — one of the differences between closed and open loops that only shows up under sustained load.

The diagnostic thread is the same in both families. A machine that frosts, clears and carries on is working. A machine drifting steadily colder — an outdoor coil losing ground to the frost, or a loop losing ground to the season — is being asked for more than its heat source can supply, which is also what settles how the two behave in extreme cold.