Heat Pumps in Cold Climates: Performance Below −15 °C

The objection you hear most often is that heat pumps “don’t work when it’s really cold”. The ownership figures say otherwise. Europe’s leaders are Norway, Sweden and Finland, where January averages sit well below freezing. Cold does change the engineering, though. Capacity drops just as heat demand peaks, efficiency falls, frost forms on the outdoor coil, and sizing choices that barely matter in a mild climate start to decide whether the system works. This guide explains what happens below 0 °C and how to design a system that stays efficient through a −20 °C cold snap.

Why cold weather reduces efficiency

Efficiency depends on the temperature lift, the gap between the heat source and the heating water. The theoretical (Carnot) limit is:

Cold weather hits twice. The lift grows as the outdoor temperature falls. At the same time the building needs more heat, so weather-compensated control raises the flow temperature. Moving from +7 °C outside / 35 °C flow to −15 °C / 50 °C roughly doubles the lift, and COP can halve as a result.

What real cold-climate units achieve

Manufacturers publish performance tables at the standard EN 14511 rating points. The figures below are typical for current inverter-driven, cold-climate air-source units (R290, R32 or vapour-injection compressors). Individual products vary by ±15%.

Gibb, Rosenow and colleagues pulled together field and lab data in a 2023 Joule review. Air-source heat pumps held COPs around 2 at −10 to −15 °C and stayed above 1.5 in most tests at −20 °C or below. So even in the worst hours a heat pump beats resistance heating, and there are not many of those hours.

Why the seasonal number holds up

Extreme cold is rare. In Oslo, Helsinki or Montréal, most heating-season hours fall between −5 °C and +8 °C. A unit with COP 1.8 at −20 °C might spend only 50–150 hours a year at that temperature. The seasonal performance factor (SPF) is weighted by the heat delivered at each temperature, so a cold site usually ends up 0.3–0.7 points below a mild one. It does not collapse.

Defrosting: the hidden loss between −7 °C and +5 °C

When the outdoor coil runs below 0 °C in humid air, frost forms and chokes the airflow. The unit then reverses the cycle for 2–10 minutes and melts it with heat taken from the house. Frosting is worst not at −20 °C, when the air is dry, but at around −3 °C to +3 °C with high humidity, which is normal weather in maritime and continental winters. In that band, defrosting usually costs 2–10% of the energy delivered. Good practice:

• Put the outdoor unit on a stand or wall bracket 30–50 cm above the expected snow line.
• Give the condensate a frost-free drain. Ice building up under the unit is a common failure.
• Keep it clear of snow drifts and roof drip lines; a small canopy helps.
• Pick a unit with demand-based (sensor-driven) defrost, not timed cycles.

Sizing for the design temperature

The design outdoor temperature is the cold-day figure the system must cope with, usually exceeded on only a few days a year. It is about −12 °C in Berlin, −14 °C to −16 °C in Bucharest, −20 °C in Oslo, −26 °C in Helsinki and around −23 °C to −30 °C in Montréal or Minneapolis. National standards and ASHRAE climate data give the exact value.

There are two approaches:

1. Monovalent sizing: the heat pump covers 100% of the design load on its own. It is simple and efficient, but the unit is oversized for most of the season.
2. Bivalent sizing: the heat pump covers the load down to a bivalent point (often −5 °C to −10 °C). Below that, an electric element or an existing boiler tops up.

The coldest hours are few, so a heat pump sized for 80–90% of peak load still supplies 95–99% of annual heat in most cases. The last few percent from a resistance heater cost little.

Worked example: 160 m² house near Oslo

• Design heat loss: 9 kW at −20 °C (renovated 1990s house); annual heat demand 21,000 kWh.
• Option A: 9 kW cold-climate air-to-water unit. At −20 °C it delivers ~70%, or 6.3 kW, and the 6 kW backup element helps out below about −12 °C. An hourly weather simulation gives backup ≈ 3% of heat and SPF ≈ 2.8. Electricity: 21,000 / 2.8 ≈ 7,500 kWh.
• Option B: ground-source with 120 m of borehole (two 60 m wells in typical Norwegian bedrock), SPF ≈ 3.8. Electricity ≈ 5,530 kWh.
• At NOK 1.5/kWh (~€0.13), Option B saves ~2,000 kWh, about €260/year, but costs €10,000–15,000 more. That is a long payback, and it is why air-source dominates new Norwegian installations despite the cold.
• Next to direct electric heating, which would use 5,000–20,000 kWh more, both options are a big step.

Try your own climate in the heat pump calculator and size boreholes with the ground source heat pump calculator.

Air-to-air: the cold-climate workhorse

In Scandinavia and Japan the typical heat pump is not a hydronic system. It is a plain air-to-air split unit on the living-room wall. Models built for Nordic markets carry SCOP ratings of 4.5–5.5 in the “colder” EU reference climate and run down to −25 °C or −30 °C. In a house heated by resistance heaters, one €2,000–3,500 unit serving the open-plan living area can cut heating electricity by 30–50%. The limits: no hot water, and closed bedrooms get little of the heat.

Ground-source and other alternatives

Below about 2 m, the ground stays close to the annual mean air temperature: roughly +5 to +8 °C in Scandinavia and +9 to +12 °C in central Europe. A borehole collector returns brine at about 0–5 °C even in February. The heat pump therefore works against an almost constant lift and never defrosts. The downsides are cost and long-term sizing: drilling runs €40–80 per metre in Europe, and boreholes must be big enough not to deplete the ground over decades.

Other options:

• Hybrid heat pumps combine an air-source unit with an existing gas or oil boiler that takes the coldest hours. They make sense where the grid is constrained or the radiators can’t be upgraded.
• CO₂ (R744) heat pumps make 65–90 °C water efficiently, which suits hot water production in cold climates.
• Exhaust-air heat pumps recover heat from the ventilation air in airtight Nordic homes.

Grid impact of cold snaps

Once millions of homes heat with heat pumps, the coldest hours set peak electricity demand. A COP-2 heat pump draws half the power of a resistance heater for the same heat. Replacing direct electric heating (common in Norway, Sweden, France and Quebec) with heat pumps therefore lowers winter peaks, while replacing gas raises them. Well-insulated homes with thermal mass can ride out a peak hour with the heating off, and smart tariffs increasingly pay for that. Compare national grid mixes on the world energy map.

Checklist for a cold-climate installation

• Do a proper heat-loss calculation at the local design temperature. Don’t copy the old boiler size.
• Pick a model with published capacity and COP data at −15 °C and −20 °C. In North America, NEEP’s cold-climate list is a good filter.
• Aim for the lowest possible flow temperature: 35–45 °C at design conditions, where the radiators allow it.
• Insulate before you buy. In a cold climate every kW of heat loss you avoid saves both equipment cost and electricity (see the insulation savings calculator).
• Mount the outdoor unit above the snow, sheltered from the prevailing wind, with a heated or frost-protected drain.
• Set the backup heater to switch on only below the bivalent point, and log what it uses.
• Compare total costs in the heating cost comparison, counting the gas standing charges you can drop.

Bottom line

Heat pumps lose capacity and efficiency as temperatures fall. Modern cold-climate units still deliver about two units of heat per unit of electricity at −15 °C, and good installations in Nordic climates reach seasonal performance well above 2.5. What makes the difference is design: sizing at the real design temperature, low flow temperatures, a well-placed outdoor unit and a backup heater that really is only a backup. For the basics of COP and running costs, read Heat Pumps Explained.

Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.