Yes. Air source heat pumps heat reliably below freezing: standard units work to −15 °C and EVI (enhanced vapour injection) models to −25 °C, still delivering a COP of about 2 even in deep cold. Air at −18 °C holds roughly 85% of the extractable heat energy it holds at +21 °C. Capacity falls as temperature drops, so cold-climate sizing and defrost management decide real-world performance.
How can a heat pump extract heat from freezing air?
Heat exists in any air above absolute zero (−273 °C); freezing point is not a physical boundary for heat extraction. Air at −18 °C still contains about 85% of the thermal energy it holds at +21 °C. The refrigerant in the outdoor coil evaporates several degrees below the outdoor air temperature — always colder than the winter air — so heat keeps flowing from air to refrigerant. This is proven at scale: Norway, Sweden and Finland, Europe's coldest markets, are also its most saturated, with Norway operating on the order of 60 heat pumps per 100 households (EHPA).
What happens to COP and capacity when temperatures drop?
Efficiency and output both decline as the gap between outdoor air and flow temperature widens. Typical values for a quality air-to-water unit at 35 °C flow temperature:
| Outdoor temperature | Typical COP (W35) | Heating capacity vs rated |
|---|---|---|
| +7 °C (A7) | 4.0–4.8 | 100% |
| +2 °C (A2) | 3.3–3.8 | 85–90% |
| −7 °C (A-7) | 2.5–3.0 | 70–80% |
| −15 °C (A-15) | 2.0–2.4 | 55–70% |
Even a COP of 2.0 means the coldest nights cost half as much as direct electric heating. Because such nights are rare, they barely dent the seasonal average: the SCOP of a well-planned system stays in the 3.5–4.5 range, since mild weather dominates the heating year. Solimpeks Varm Up units reach heating COPs of up to 4.9 at A7/W35, and the seasonal figure is what utility bills follow.
What is the defrost cycle and why does it matter in winter?
Between roughly +5 and −7 °C, humid air freezes onto the outdoor coil and throttles airflow. The unit periodically runs a defrost cycle — briefly reversing the circuit to melt the ice using heat drawn from the heating water. This is normal, but it is why manufacturers demand a minimum system water volume, usually met with a buffer tank of 10–20 L per kW: without that thermal reserve, defrosts chill the house and trip low-temperature alarms. Steam rising from the outdoor unit during defrost is melting frost, not smoke.
What is an EVI compressor and when do you need one?
An EVI compressor (enhanced vapour injection) injects partly expanded refrigerant mid-compression, cooling the compressor and extending the working envelope deep below freezing. The practical result is 20–30% more heating capacity at −15 °C and stable operation to −25 °C with flow temperatures up to 60–65 °C. Solimpeks Varm Up EVI models, rated for operation down to −30 °C, use exactly this technology for continental climates such as central Anatolia, the Balkans and eastern Europe. In the harshest zones, installers may still combine the heat pump with a small backup in a bivalent system, sized so the backup covers only the coldest 1–5% of annual hours.
How do you design a system for cold climates?
Four rules cover it: size from a proper heat load calculation at the local design temperature, not a rule of thumb; keep the flow temperature as low as the emitters allow, because each 1 °C saved adds roughly 2–3% efficiency; provide defrost volume; and mount the outdoor unit 20–30 cm above grade with free drainage so meltwater never refreezes into the coil. Done properly, heat pumps out-heat their reputation — 2025 European sales grew 13% to 2.88 million units (EHPA), with cold-climate markets leading adoption per capita.
Frequently asked questions
At what temperature do heat pumps stop working?
Standard air source units typically operate to −15 °C and EVI models to −25 °C; below the cut-off the unit stops and any backup takes over. In most of Europe such temperatures occur rarely enough that a correctly sized unit never reaches its limit.
Is a ground source heat pump better for very cold regions?
Ground temperature is stable at 8–12 °C year-round, so ground source units keep high COP regardless of air temperature. They cost significantly more to install, so modern EVI air source units are usually the more economic choice except in extreme climates.
Do I need a backup heater below freezing?
Not in most European climates if the unit is sized to the design heat load. In continental cold zones a small electric element or existing boiler covering the few coldest days keeps the heat pump smaller and cheaper without compromising comfort.
Why does my heat pump steam and drip water in winter?
That is the defrost cycle melting frost off the outdoor coil — normal operation. Ensure the condensate can drain freely so it does not refreeze under the unit.