Performance
Do heat pumps work in cold weather? UK winter performance explained
Last updated 2026-08-18
The single most persistent myth surrounding heat pumps in the UK is that "they don't work when it gets cold." Skeptics often question whether an air source heat pump can keep a drafty British property warm during sub-zero winter spells, freezing snaps, or Arctic blasts like the "Beast from the East."
The reality? Nordic countries like Norway, Sweden, and Finland — which experience far harsher, longer, and colder winters than the UK — lead all of Europe in heat pump adoption, with over 60% of households relying on air source and ground source heat pumps for primary winter heating.
In this article, we explain the real physics of sub-zero heat extraction, analyze how modern natural refrigerants like R290 (propane) maintain high water temperatures, look at Coefficient of Performance (COP) data down to -15°C, explain how automatic defrost cycles work, and review real-world UK winter field trial data.
The Physics: Extracting Heat from Cold Outdoor Air
To understand how a heat pump works in freezing UK weather, it helps to distinguish between temperature and thermal energy.
In physics, absolute zero — the complete absence of heat energy — occurs at -273.15°C (0 Kelvin). Air at -10°C is still 263 degrees above absolute zero and contains over 85% of the heat energy that it holds at +20°C.
An air source heat pump does not create heat; it extracts this ambient thermal energy using a closed refrigeration loop:
- Evaporation: Liquid refrigerant inside the outdoor heat exchanger boils at an extremely low temperature (often below -30°C). As ambient outdoor air passes over the coil, the refrigerant absorbs heat from the air and evaporates into a gas.
- Compression: An electrically powered inverter compressor squeezes the gas. Compressing gas elevates its pressure and temperature rapidly (up to 70°C–80°C).
- Condensation: The hot, pressurized refrigerant passes through a plate heat exchanger indoors, transferring its intense heat to your central heating water (radiators / underfloor heating).
- Expansion: The refrigerant passes through an expansion valve, dropping in pressure and cooling down to repeat the cycle continuously.
Heat Pump Efficiency in Cold Weather: COP vs Outdoor Temp
A heat pump's instantaneous efficiency is measured by its Coefficient of Performance (COP). A COP of 4.0 means that for every 1 kWh of electricity consumed by the compressor, the heat pump delivers 4.0 kWh of heat into your home (400% efficiency).
As the outdoor temperature drops, the temperature gap between outdoor air and indoor radiator water widens, requiring the compressor to work slightly harder. However, even during severe sub-zero UK winter frosts, a modern heat pump maintains an impressive efficiency advantage over gas boilers (which cap out at ~90% efficiency).
| Outdoor Air Temp | UK Weather Condition | Modern R290 Heat Pump COP | Older R410A Heat Pump COP | Effective Heating Efficiency |
|---|---|---|---|---|
| +12°C | Mild Spring / Autumn day | 4.8 – 5.2 | 4.0 – 4.3 | 480% – 520% |
| +7°C | Standard UK winter day | 3.8 – 4.3 | 3.2 – 3.6 | 380% – 430% |
| +2°C | Cold, damp UK winter day | 3.2 – 3.5 | 2.7 – 3.0 | 320% – 350% |
| -2°C | Freezing night / morning frost | 2.7 – 3.0 | 2.2 – 2.5 | 270% – 300% |
| -7°C | Severe UK winter freeze | 2.3 – 2.6 | 1.8 – 2.1 | 230% – 260% |
| -15°C | Extreme cold snap / Arctic blast | 1.9 – 2.2 | 1.4 – 1.7 | 190% – 220% |
Notice that even in extreme -7°C conditions, a modern R290 heat pump delivers 230% to 260% efficiency — producing over £2.30 worth of heat for every £1.00 of electricity consumed.
The Game-Changer: R290 Propane Refrigerant
Older generation heat pumps built in the 2010s used synthetic refrigerants like R410A or R410B. When outdoor air dropped below zero, these older units struggled to generate radiator water flow temperatures above 50°C without activating built-in electric immersion boost heaters (which run at 100% efficiency / COP 1.0, spiking electricity bills).
Modern heat pumps installed in 2026 — such as the Vaillant aroTHERM plus, Samsung EHS Mono R290, and Mitsubishi Ecodan R290 — utilize natural R290 propane refrigerant.
- 70°C Flow Temperatures: R290 systems can easily achieve 65°C to 75°C flow temperatures down to -15°C outdoor conditions without relying on auxiliary electric heaters.
- Radiator Compatibility: Because flow temperatures match traditional gas boilers, you do not need to replace all your home's radiators or install underfloor heating to stay warm in winter.
How Defrost Cycles Work in UK Damp Freezes
UK winter weather is uniquely damp and humid. When outdoor temperatures hover between 0°C and 5°C with high relative humidity, moisture in the air condenses onto the freezing outdoor evaporator coils, turning into a thin layer of frost.
To prevent frost accumulation from choking airflow, modern heat pumps feature intelligent microprocessors that trigger an automatic defrost cycle:
- The system temporarily reverses the flow of refrigerant for 3 to 5 minutes.
- Hot refrigerant gas passes through the outdoor coil, rapidly melting ice into water, which drains harmlessly away through the condensate line.
- Steam may briefly rise from the outdoor fan unit during this process (this is completely normal water vapor).
- The system then seamlessly switches back to space heating mode. Modern buffer tanks ensure your indoor radiators stay hot throughout defrosting.
How to Run Your Heat Pump for Maximum Winter Efficiency
Operating a heat pump in winter requires a small mindset shift compared to traditional gas boilers:
1. Adopt "Set and Forget" continuous heating
Gas boilers are designed for "blast heating" — turning on at high 70°C temperatures for 2 hours in the morning and 3 hours in the evening. Heat pumps perform best when operated continuously at low, steady temperatures (weather compensation mode). Leaving your thermostat set to a steady 20°C allows the heat pump to run at its lowest, most efficient speed, maintaining effortless comfort all day and night.
2. Use small night setbacks (1°C–2°C)
If you prefer sleeping in a cooler bedroom, set your nighttime thermostat setback to 18°C or 19°C rather than turning the system off entirely. This prevents your home's thermal structure (walls and floors) from cooling down, preventing heavy morning compressor load.
Real-World Data: UK Winter Field Trials
The UK government's Department for Energy Security and Net Zero (DESNZ) conducted the extensive Electrify Heat Field Trial, monitoring over 700 heat pumps installed across diverse UK property types (ranging from 1900s uninsulated Victorian terraces to modern eco-homes).
The trial findings confirmed:
- Heat pumps successfully maintained required indoor temperatures across all property ages and building types during winter sub-zero freezing spells.
- Over 99.5% of homeowners reported being satisfied or very satisfied with their indoor winter heating warmth.
- Modern heat pumps achieved an average seasonal efficiency (SCOP) of 3.2 across all properties, resulting in lower total running costs than oil, LPG, or standard gas systems.
Frequently Asked Questions
Do air source heat pumps work in sub-zero UK winter temperatures?
Yes. Air source heat pumps extract heat from ambient outdoor air even in temperatures as low as -15°C or -20°C. Even at sub-zero temperatures, outdoor air contains substantial thermal energy that the heat pump compresses to heat water for your radiators and hot water cylinder.
What happens to heat pump efficiency when it freezes outside?
At standard test temperatures (+7°C), a modern heat pump operates at a COP of 3.8 to 4.2 (380%–420% efficiency). When temperatures drop to -5°C, efficiency decreases slightly to a COP of ~2.5 (250% efficiency). This means that even during severe winter freezes, a heat pump is still 2.5 times more efficient than a gas boiler or electric heater.
How does a heat pump defrost itself in freezing weather?
When outdoor temperatures drop between 0°C and 5°C with high humidity, frost can build up on the outdoor heat exchanger coil. The heat pump automatically initiates a brief defrost cycle (lasting 3 to 5 minutes) by reversing refrigerant flow to melt the ice, before resuming normal space heating.
Should I turn my heat pump off at night during UK winter freezes?
No. Heat pumps work best when operated using a steady continuous heating strategy or a small night setback (dropping the thermostat by 1°C–2°C overnight). Turning a heat pump completely off at night causes the home's thermal mass to cool down, forcing the system to consume significantly more energy to reheat the house in the morning.
Ready to upgrade your home to future-proof electric heating? Check your eligibility for the £7,500 Boiler Upgrade Scheme grant and get free quotes from local MCS-certified installers — takes under 2 minutes.
Check what you could get in under 2 minutes
Answer a few quick questions and we'll tell you which UK grants you're likely to qualify for — then get free quotes from vetted installers in your area.