Comparison · 6 min read · Updated 2026-05-07
Heat pumps in cold climates — what changes below -15 °C
Cold-climate HPs work fine to -25 °C if you choose right. The matrix of refrigerants, defrost strategies and backup heating that makes Nordic deployments succeed.
The myth
You'll still see "heat pumps don't work in cold climates" online. The data: Norway has the highest heat-pump density per capita in the world — over 70% of detached homes. Finland is at 50%+. Both countries spend February at -15 to -25 °C routinely.
Modern HPs work fine. What changes is how you specify them.
What gets harder below 0 °C
Three physical problems:
1. Outdoor coil frosts. Humidity in the air condenses and freezes on the evaporator. The unit must reverse cycle periodically to melt it (defrost), spending energy and dropping output. 2. Compressor work increases. The temperature lift between source (-15 °C) and sink (35 °C low-temp / 55 °C high-temp) widens. 3. Air gets thinner / energy-poorer. Per cubic metre, -15 °C air carries less heat than +5 °C air. The fan has to move more.
Result: COP at A-15/W35 might be 2.5 where A2/W35 was 4.5. Still beats resistive heat (COP 1) by a wide margin, but the compressor runs longer and your annual SCOP for a cold climate is 3.0–3.5, not 4.5.
What modern cold-climate HPs do differently
- Larger outdoor units with bigger evaporator surface — defrost
less often, lose less to frost.
- EVI (Enhanced Vapour Injection) or two-stage compressors —
maintain capacity at low ambient temperatures instead of derating to 50% at -15 °C.
- R290 propane as refrigerant — superior thermodynamics at low
temperatures vs R32, which loses pressure ratio efficiency below -10 °C.
- Inverter-driven control — modulates instead of cycling, keeping
COP smooth across the season.
Look for spec sheet entries:
- Capacity at A-15/W35 ≥ 80% of nominal
- Defrost cycles using reverse cycle (not resistive)
- Compressor approved for operation to -25 °C
Sizing for cold climates
Two extra checks beyond the standard sizing exercise:
1. Design temperature — your local building code's coldest expected day. In Stockholm it's -16 °C, in Trondheim -19 °C, in Tromsø -22 °C. The HP must deliver full design heat load there without the electrical backup heater kicking in. Backup heating is fine for emergency, but if your unit needs it on every cold week, your SCOP tanks. 2. Buffer cylinder larger than usual — defrost cycles temporarily stop heating output. A bigger buffer (60–80 L per kW vs the typical 30 L) carries the home through defrost without temperature dip.
Ground-source as the cold-climate default
Above ~3 500 HDD₁₈ (anywhere in Sweden, Finland, Norway, much of the Baltics), ground-source becomes economically attractive again because:
- Source is +8 °C all year — no defrost, no derating.
- SCOP stays > 4.5 even in Helsinki vs ~3.5 for air-source.
- Subsidies often cover the borehole cost (NZÚ in Czechia, Czyste
Powietrze higher-tier in Poland, BEG climate-bonus in Germany).
Top cold-climate models in our catalog
Search our air-water leaderboard with cold-climate filter for SCOP > 4 in Average climate (those usually retain ≥ 3.5 in Cold). NIBE, IVT, CTC, Mitsubishi Ecodan, Daikin Altherma 3 H HT, Bosch Compress 7800i — all have proven Nordic deployments.
When NOT to go HP-only
If your house has:
- Single-glazed windows + no insulation
- Existing radiators sized for 80 °C flow
- Climate below -25 °C routinely
…then either (a) renovate the envelope first, or (b) install a hybrid system with kept fossil backup for the worst weeks. A heat pump running flat-out at -28 °C with backup electric resistive heater is not better than a modern condensing oil boiler.
This is rare in 2026 — most cold-climate homes have been upgraded. But it's the honest exception.