Heat pump freezing up or constantly defrosting: what is normal and what is not

Short answer: a layer of frost on the outdoor unit is perfectly normal in freezing weather; the heat pump defrosts itself periodically, and that may happen a few times a day. What is not normal is a unit that freezes up completely, defrosts every few minutes, or does not get back up to temperature after defrosting. Then the airflow is obstructed, a sensor is measuring incorrectly, or there is too little refrigerant in the system.

Why a heat pump freezes at all

The outdoor unit extracts heat from the outside air. As a result, the evaporator cools to below the outside temperature, and in humid weather the moisture in the air settles on it as frost. This happens most between about zero and seven degrees with high humidity; in severe frost the air is actually drier and it is less of a problem.

A layer of frost insulates and worsens heat transfer, so the heat pump periodically reverses the process: it briefly takes heat from the home to defrost the evaporator. This takes a few minutes, you sometimes hear a hissing or clicking sound and steam comes off the unit. That is all part of it, even if it looks as if something is going wrong.

When it really is a problem

Watch for three patterns. First: the unit defrosts much more often than normal, for example every quarter of an hour. Second: the ice no longer disappears completely and builds up over the days until the block is iced shut. Third: the base tray or the drain is full of ice, so the meltwater cannot drain away and immediately freezes again. The latter is a classic with units that are placed too low or in a closed enclosure.

First check the simple things: is the unit free-standing, is there at least half a metre of space around it, is there no leaves or dirt against the fins, and can the meltwater drain away underneath? A silted-up drain or a unit tucked away behind a fence causes more ice problems than any defect.

The messages by brand

Brand	Code	Message
Remeha	H06.06	Compressor lockout, high pressure
Remeha	H06.07	Compressor lockout, low pressure
Itho Daalderop	A1 21	Compressor starting current too high or temperature protection triggered
Vaillant	F.279	Hot gas thermostat switches off the heat pump

A low-pressure message such as H06.07 often points to too little refrigerant or an obstructed evaporator; a high-pressure message, on the other hand, to too little heat output on the indoor side, for example due to a clogged filter or too little flow in the system. All codes by brand can be found in the fault codes for heat pumps.

No more heat, without ice

If the heat pump no longer reaches temperature at all, first check the water pressure of the heating circuit and whether the dirt filter in the return pipe is clean. Heat pumps are much more sensitive to contamination than boilers: they work with small temperature differences, so any restriction in flow is immediately noticeable in the heat output.

Then check whether the appliance has remained in a safety mode after a power cut; some units then have to be released manually. And check that the electric backup heater has not been switched off, because at low outside temperatures the system partly relies on it.

What to leave to an engineer

Anything involving refrigerant. Work on refrigerant circuits must be carried out by a certified refrigerant (F-gas) technician; Instalpunt does not install, so have a qualified local installer do the work. Moreover, topping up refrigerant is not maintenance but a repair: a system does not consume refrigerant, so if the level is too low, there is a leak that must be traced first.

What you can do yourself: keep the outdoor unit clear, remove leaves, check the drain and clean the dirt filter indoors. Never pour hot water over a frozen unit and do not chip the ice off; the fins and sensors are fragile.

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Frequently asked questions

How often may a heat pump defrost?

In humid weather around freezing point, several times a day, with cycles of a few minutes. If it defrosts every quarter of an hour, something is wrong: too little airflow, a dirty evaporator or a sensor that measures incorrectly.

Can I spray the ice off with warm water?

Preferably not. Lukewarm water can be used as an emergency measure on a completely frozen unit, but the water then freezes again in the drip tray. The unit also contains electrical parts. Have an engineer establish the cause first.

Why is my electricity consumption so high in winter?

At low outdoor temperatures efficiency drops and the electric backup heater kicks in more often. That is normal, but a poorly balanced installation or a flow temperature that is too high makes the effect considerably worse. Have the settings checked if consumption differs greatly from previous winters.

Mijn buitenunit maakt meer lawaai dan vroeger. Erg?

Usually it is the fan: dirt on the impeller or worn bearings. Sometimes vibrations are transmitted through the pipework into the house. Parts are listed under fans; also check whether the anti-vibration mounts under the unit are still in good condition.