A heat pump also heats reliably at severe sub-zero temperatures – it simply needs more electricity then because the source is cold. Steam clouds and short defrost phases on the outdoor unit are normal operation, not a defect. You should check little in winter, but in a targeted manner: clear airways, functioning condensate drain – and whether the immersion heater only runs exceptionally. (As of: July 2026)
Key points in brief
- Modern air-to-water heat pumps are designed for the Swiss climate and heat even at double-digit sub-zero temperatures – geothermal probes hardly notice winter anyway.
- The steam cloud on the outdoor unit is water vapour from defrosting, not smoke and not a fault.
- Higher electricity consumption in winter is physics: cold source, large temperature lift – the annual performance factor is calculated over the year, not in January.
- Your winter checklist is short: keep airways clear, check condensate drain, no snow castle around the unit.
- Warning signals are continuous running of the immersion heater, a layer of ice instead of frost – and consumption that is significantly higher than in previous winters.
Does the heat pump also work in severe frost?
Yes – and by design, not by luck. Modern air-to-water heat pumps are designed for operation at significant sub-zero temperatures; even cold winter air contains heat energy that can be utilised. What changes as the temperature drops is not the if, but the effort: the colder the source, the larger the temperature lift, the more electricity the same heat needs. The system comparison explains why the geothermal probe does not have this problem – its source remains almost constant all year round, its winter does not take place.
For owners of an air-to-water system, this means in practice: January is the most expensive heating month, and that is priced in. The annual performance factor – the efficiency metric of the system – is an annual average in which the cold weeks are already included. Anyone who looks at winter consumption in isolation and is shocked makes the same error in reasoning as with the solar yield in December: single months are no use as a benchmark.
The steam cloud and defrosting: normal operation, no defect
The sight unsettles everyone in the first winter: a white cloud rises from the outdoor unit, sometimes for minutes at a time. This is not smoke and not a leak, but water vapour – and it is part of the system's cleverest trick, defrosting.
The background: if the unit draws damp, cold air over the heat exchanger, frost forms there – physically inevitable, especially at temperatures around freezing point with high humidity. To prevent the frost from blocking the airflow, the system regularly defrosts itself: it briefly reverses the process, melts the coating – and this creates the steam cloud. After that, heating operation continues. These cycles cost some efficiency (this is also already included in the annual performance factor) and are more frequent in wet snow weather than in bitter cold – dry polar air freezes less than damp zero-degree soup.
What is normal: frost on the heat exchanger, occasional steam clouds, water under the unit. What is not normal is a layer of ice that remains – more on this below.
Your winter checklist: three simple steps, no more
The heat pump needs no supervision in winter – but you should keep an eye on three things, all from the ground and without tools:
1. Keep airways clear. The outdoor unit must be able to draw in and blow out air unhindered. Autumn leaves, blown snow, the snow blower embankment or the decorative cladding that didn't bother anyone in summer – anything that restricts the airflow depresses efficiency. After heavy snowfall, a quick look around the unit is worthwhile; shovelling it free, yes, but at a distance and without touching the fins. 2. Check condensate drain. The defrost water must be able to drain away. If the drain freezes over, the water backs up and freezes where it shouldn't – the most common cause of the ice problems that are then falsely blamed on the unit. A glance after cold nights is enough. 3. Lean nothing against it, put nothing over it. Well-intentioned «winter covers» suffocate the unit. The system is built for weather – what it needs is air, not protection.
Everything beyond this – cooling circuit, electrics, settings in the service menu – is a matter for the specialist company, in winter as in summer.
Warning signals: when you should call the specialist company
The heating element runs constantly. The additional electrical heating element is there for the few extreme days; the fact that it occasionally steps in is planned. However, if it runs regularly or for weeks – recognisable from the monitoring or simply an electricity bill that is out of the ordinary –, something is wrong: too tight a dimensioning, an incorrectly adjusted control or a technical problem. This needs to be looked at, not ignored.
Ice instead of frost. A thin layer of frost that disappears in the defrost cycle is an everyday occurrence. A growing armour of ice on the device, frozen fins or icicles from the casing show that the defrosting can no longer keep up or the condensate is not draining – a clear case for servicing.
Significantly more consumption than in previous winters. The fair basis for comparison is the winter before, not October. If the consumption is markedly higher in similar weather, it is worth looking at the heating curve, defrost behaviour and heating element – the same comparison logic as for solar monitoring: December with December.
New noises. A fan that sounds a bit more present in winter is normal – it is working harder. Grinding, rattling or a humming that was not there last winter, on the other hand, are worth a call; also out of consideration for the neighbourhood.
What winter means for electricity and the solar system
Winter is the month of inconvenient truth for the combination with photovoltaics: The heat pump needs the most when the roof delivers the least. This is no argument against the combination – over the year the balance remains clearly positive –, but it calibrates expectations: The winter electricity for the heat pump comes predominantly from the grid, at the tariff that your utility has for heat pumps. Whoever plans both, plans it with this honesty – and is happy for it in the transition period, when heating operation and solar yield actually meet.
The most frequent winter call we receive about heat pumps is not a defect, but a photo: the cloud of steam, taken by the worried owner or – more often – by the neighbour who has seen «smoke». Since we say a sentence about it at handover («In winter the device steams from time to time – that is water vapour from defrosting»), these calls have practically disappeared. The second most frequent case is more serious and almost always the same: ice on the device, and the cause does not lie in the cooling circuit, but a floor lower – a frozen or blocked condensate drain, sometimes simply a pile of leaves from autumn. Five minutes of checking in December saves the service appointment in January here. And the third classic is the most reassuring: «The system is running continuously today – is that bad?» No. That is exactly how a correctly dimensioned heat pump should work on the coldest day: quietly, evenly, without a pause – and without a heating element.
Frequently asked questions
Down to what outside temperature does an air-to-water heat pump work?
Modern devices are designed for the Swiss climate and heat reliably even in severe, double-digit sub-zero temperatures – the operating limits are lower than our Swiss Plateau winter ever gets. What rises with the cold is the electricity consumption, not the risk of failure.
Is the white cloud on the outdoor unit dangerous?
No – that is water vapour from the defrost cycle, with which the device melts frost from the heat exchanger. No smoke, no refrigerant, no defect. It occurs more frequently in damp weather around freezing point; in dry cold less freezes over.
Should I cover the outdoor unit in winter?
Under no circumstances – covers block the airflow that the device needs to work, and promote exactly the icing that they are supposed to prevent. The system is built for wind and weather; only intake, exhaust and condensate drain must remain free.
The heating element sometimes kicks in – is my system too small?
Not necessarily: On the few extreme days, the use of the heating element is planned and unproblematic. A warning signal is only regular or continuous operation – then dimensioning and settings need to be checked. The monitoring or the electricity bill show the difference.
Should I manually increase the flow temperature in winter?
No – that is exactly what the heating curve is for: It automatically raises the flow temperature when it gets colder outside. Manual readjustment permanently alters the control and costs efficiency. If individual rooms do not get warm, this is a case for hydronic balancing, not for the temperature controller.
Free initial consultation
So that winter remains routine.
We set the heating curve and defrost behaviour correctly, check the installation and condensate drain – and explain at handover what is normal in winter. Even for systems that were not built by us.
Prefer to talk? +41 78 830 83 35
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Swissolar-certified specialist company · ESTI installation authorisation (Art. 14 NIV) · in Zurich since 2017 · over 150 systems completed · a personal answer from the specialist company, no call centre
Sources: Fachvereinigung Wärmepumpen Schweiz FWS (operating principles); empirical values from the service and installation practice of ecoEn GmbH, Zurich region.
Last updated: 9 July 2026 · Author: ecoEn editorial team
