"Heat Pumps Don't Work in Cold Weather" — The Data Says Otherwise

It is the single most common objection to heat pumps in the UK: "They don't work when it gets cold." You will hear it from neighbours, read it in newspaper comment sections, and see it repeated endlessly on social media. The problem is that it is completely wrong. Not just a little bit wrong — comprehensively, demonstrably, provably wrong. And we have the data to prove it.

In this article, we examine real-world performance data from countries that experience winters far harsher than anything the UK ever faces. We look at laboratory testing from independent bodies. We analyse monitoring data from UK installations. And we explain the science behind why modern air source heat pumps are engineered to thrive in cold conditions — not just survive them.

Where This Myth Comes From

The idea that heat pumps cannot handle cold weather has three main origins, and understanding them helps explain why the myth persists despite overwhelming evidence to the contrary.

First, outdated technology. Early air source heat pumps from the 1990s and early 2000s did struggle in cold weather. They used fixed-speed compressors that could not modulate output, and their refrigerants were less effective at low temperatures. A heat pump from 2005 and a heat pump from 2026 are fundamentally different machines. Judging modern heat pumps by the performance of 20-year-old models is like judging electric cars by the range of a 2010 Nissan Leaf.

Second, fossil fuel industry messaging. As we explore in our article on the gas boiler lobby, there has been a concerted effort to undermine public confidence in heat pumps. The "cold weather" argument is the single most effective tool in this campaign because it sounds intuitively plausible. Of course a machine that extracts heat from outside air would struggle when that air gets cold — except the science does not work that way.

Third, confusion between efficiency and functionality. Heat pumps do become less efficient as temperatures drop. A unit delivering a COP of 4.0 at 10°C might deliver 2.5 at -5°C. Some commentators present this efficiency reduction as "not working." But a COP of 2.5 still means producing 2.5 units of heat for every unit of electricity — far more efficient than any gas boiler or electric heater at any temperature.

The Scandinavian Evidence: Millions of Heat Pumps in Extreme Cold

If heat pumps did not work in cold weather, Norway, Sweden, and Finland would not have installed millions of them. The evidence from these countries is not just compelling — it is conclusive.

Norway: The World Leader

Norway has over 1.3 million heat pump installations for a population of just 5.5 million — roughly one heat pump for every four people. According to the Norwegian Heat Pump Association (NOVAP), heat pumps supplied approximately 37 TWh of heating energy in 2024. Norwegian winters routinely see temperatures of -20°C to -30°C in inland areas, with the far north experiencing even colder conditions.

The average Norwegian heat pump achieves a seasonal COP of 2.8-3.2, according to data from SINTEF, Norway's largest independent research organisation. This is across the full heating season, including the coldest months. In a country where January average temperatures range from -3°C on the coast to -15°C inland, these are remarkable numbers.

Sweden: Deep Cold, High Performance

Sweden has over 700,000 heat pump installations and some of the coldest inhabited areas in Europe. The Swedish Energy Agency monitors heat pump performance closely. Their data shows seasonal performance factors (SPF) of 2.5-3.5 across the country, including northern regions where winter temperatures regularly reach -25°C.

Stockholm, with average January temperatures of -3°C (similar to Edinburgh), sees seasonal COP values of 3.0-3.5 from modern installations. Northern cities like Umeå (-10°C average in January) still achieve seasonal COP values above 2.5.

Finland: Over 1 Million Installations in Arctic Conditions

Finland crossed 1 million heat pump installations in 2023, according to the Finnish Heat Pump Association (SULPU). Finland's capital Helsinki has average January temperatures of -5°C, while Rovaniemi on the Arctic Circle averages -13°C in January. Despite these conditions, heat pumps are the fastest-growing heating technology in the country.

UK Performance Data: What Monitoring Studies Show

We do not need to rely solely on Scandinavian evidence. The UK has its own comprehensive monitoring studies that prove heat pumps work effectively in British winters.

The Electrification of Heat Demonstration Project

The largest UK heat pump trial to date, led by Energy Systems Catapult and funded by BEIS (now DESNZ), monitored 742 heat pump installations across the UK over multiple heating seasons. The results, published in 2024, showed an average seasonal performance factor (SPF) of 2.78 across all installations — with the best-performing quartile achieving SPF values above 3.2.

Critically, the study found no significant performance cliff during cold snaps. During the December 2022 cold spell, when temperatures dropped to -10°C in parts of the UK, monitored heat pumps continued operating with COP values of 2.0-2.5. They kept homes warm. They did not fail. They did not need backup.

Nesta's Monitoring Data

Nesta has monitored hundreds of UK heat pump installations and published extensive data. Their analysis shows that while COP does decrease in colder weather, the reduction is gradual and predictable — not the catastrophic failure that myth-makers suggest. Even during the coldest UK days, heat pumps delivered warmth reliably.

The Science: Why Cold Air Still Contains Usable Heat

Understanding why heat pumps work in cold weather requires a brief look at the thermodynamics involved. The key insight is this: even very cold air contains enormous amounts of thermal energy.

Air at -15°C is not "cold" in an absolute sense. On the Kelvin scale, -15°C is 258K — still containing roughly 94% of the thermal energy present at 20°C (293K). A heat pump's refrigerant boils at temperatures far below -15°C (R290 propane boils at -42°C at atmospheric pressure), so there is always a temperature differential that allows heat extraction.

The thermodynamic process works the same at -15°C as it does at +15°C:

1. Evaporation: Liquid refrigerant absorbs heat from the outside air and evaporates into gas. This happens because the refrigerant's boiling point is far below the air temperature.
2. Compression: The compressor pressurises the gas, dramatically increasing its temperature. Even gas that evaporated at -20°C can be compressed to temperatures above 60°C.
3. Condensation: The hot, high-pressure gas releases its heat into your heating system and condenses back to liquid.
4. Expansion: The liquid passes through an expansion valve, dropping in pressure and temperature, ready to absorb heat again.

The physics of this cycle does not break down at low temperatures. Efficiency reduces because the compressor must work harder to achieve the same temperature lift, but the process remains fundamentally effective. As the Energy Saving Trust notes, modern air source heat pumps are designed to extract heat from air as cold as -25°C.

COP at Different Temperatures: The Real Numbers

Here is what actual performance looks like across a range of outdoor temperatures. These figures are based on laboratory testing and real-world monitoring data from multiple sources.

Sources: Energy Systems Catapult monitoring data; manufacturer specifications; SINTEF laboratory testing. COP values assume flow temperature of 35-45°C.

The critical point from this table: even at -20°C — a temperature the vast majority of the UK has never experienced — a heat pump still produces 1.5-2.0 units of heat for every unit of electricity. A gas boiler, by comparison, produces at most 0.92 units of heat per unit of gas regardless of the outdoor temperature. A direct electric heater produces exactly 1.0 unit of heat per unit of electricity at any temperature. The heat pump outperforms both alternatives at every single temperature point.

Modern Technology That Makes It Work

Today's heat pumps incorporate several technologies specifically designed for cold weather performance. These were not available in the older models that gave heat pumps an undeserved reputation for cold-weather weakness.

Inverter-Driven Compressors

Modern heat pumps use variable-speed (inverter) compressors that adjust their speed to match demand. In cold weather, the compressor speeds up to maintain output. In milder conditions, it slows down to save energy. This contrasts with older fixed-speed compressors that could only run at full power or not at all. The difference in cold-weather performance is dramatic.

Enhanced Vapour Injection (EVI)

Many modern heat pumps — including models from Mitsubishi, Daikin, and Samsung — use enhanced vapour injection technology. EVI injects additional refrigerant vapour into the compressor during cold weather operation, boosting output and efficiency. It is one of the key technologies that makes modern heat pumps dramatically better in cold weather than their predecessors.

R290 Propane Refrigerant

The newest generation of heat pumps uses R290 (propane) refrigerant, which has excellent thermodynamic properties at low temperatures. R290 boils at -42°C at atmospheric pressure, giving it a huge temperature differential even in extreme cold. Models from Vaillant, Nibe, and Viessmann using R290 show particularly strong cold-weather performance.

The Defrost Cycle: Understanding the Process

One aspect of cold-weather operation that sometimes causes concern is the defrost cycle. When outdoor temperatures hover around 0-5°C with high humidity, frost can form on the heat pump's outdoor heat exchanger. The unit periodically reverses its cycle briefly to melt this frost.

Defrost cycles typically last 2-5 minutes and occur every 30-90 minutes during frosty conditions. During defrost, the heat pump temporarily stops heating. This is normal and accounted for in all performance data. A well-designed system with a buffer tank or sufficient thermal mass in the heating circuit means occupants do not notice any temperature drop during defrost cycles.

Interestingly, defrost is more of an issue at 0-5°C than at -10°C. At very low temperatures, the air contains less moisture, so less frost forms. The conditions that cause the most frosting — just above or around freezing with high humidity — are common in the UK but rarely extreme.

Why Proper Sizing Matters More Than Temperature

If there is one factor that genuinely affects whether a heat pump keeps a home warm in cold weather, it is not the outdoor temperature — it is whether the system was properly designed and sized.

A correctly sized heat pump is calculated to meet the heat demand of the home at the design temperature — typically -3°C for southern England or -5°C for Scotland. This means at the coldest typical temperatures, the heat pump can run at or near full capacity and still meet demand. On milder days, it operates at part load, which is actually more efficient.

Problems occur when systems are undersized. An installer who cuts corners on the heat loss calculation, or who underestimates the design temperature, may specify a heat pump that struggles on the coldest days. This is not a technology failure — it is an installation failure. The same would happen if a gas boiler were undersized. Getting proper quotes from MCS-certified installers who conduct thorough heat loss surveys is essential.

The cost of a properly sized system may be slightly higher than a minimal installation, but the difference in cold-weather performance and year-round efficiency makes it a worthwhile investment.

Real UK Homes in Cold Regions

The monitoring data tells one story. Real-world experiences from UK homeowners in the coldest regions tell the same story from a different angle.

Scottish Highlands: -12°C and Still Warm

Heat pump installations in the Scottish Highlands — some of the coldest inhabited areas of the UK — consistently report satisfactory performance. The BUS grant data from DESNZ shows strong uptake in Highland and Moray council areas, where homeowners are replacing oil boilers with air source heat pumps. These are homes that regularly experience temperatures below -10°C, and the installations are working.

Northern England: Real Winter, Real Warmth

Monitoring data from installations in Northumberland, County Durham, and the Yorkshire Dales shows seasonal COP values of 2.6-3.0 — marginally lower than southern England (2.8-3.2) but still excellent. The difference is a few percent in efficiency, not a difference between working and not working.

These homes are saving significant sums compared to their previous oil or LPG heating systems, and many are also benefiting from solar panel installations to reduce electricity costs further, particularly during the longer daylight hours of spring and autumn.

Putting UK Cold in Context

To understand why the cold-weather argument is especially absurd in the UK context, consider these average January temperatures:

Source: Met Office (UK data); SMHI, MET Norway, FMI (Nordic data). Average January temperatures.

The UK is one of the mildest countries in northern Europe. Our "cold" winters are warmer than the average winter conditions in countries that have adopted heat pumps on a massive scale. The coldest day in most of the UK is a normal Tuesday in Norway.

The Comparison Nobody Makes

Here is something you will never hear from heat pump sceptics: gas boilers also lose efficiency in cold weather. When it is colder outside, your home loses heat faster, and your boiler runs harder and longer. Boiler efficiency drops during extended high-demand periods, particularly in older models. Yet nobody writes headlines saying "Gas boilers don't work in cold weather."

The double standard is revealing. A gas boiler that runs flat out during cold snaps and costs more to operate is considered normal. A heat pump that operates at slightly reduced efficiency (while still outperforming gas) is framed as a failure. This inconsistency tells you more about the motivation behind the myth than about the technology itself.

When you compare the full-year economics of heat pumps versus gas boilers, including cold weather operation, heat pumps come out ahead in the vast majority of UK homes. The running cost analysis accounts for seasonal efficiency variations.

Frequently Asked Questions

Do heat pumps work below zero?

Yes. Modern air source heat pumps operate effectively at temperatures as low as -25°C. Real-world data from Scandinavia confirms COP values of 2.0-2.5 at -15°C. The UK rarely drops below -5°C, well within comfortable operating range.

At what temperature do heat pumps stop working?

Most modern units are rated to -20°C or -25°C. Efficiency reduces gradually as temperatures drop, but they still produce 2-3 units of heat per unit of electricity even in extreme cold. They do not "stop working" at any temperature the UK experiences.

Why do people say heat pumps don't work in cold weather?

The myth comes from outdated 1990s/2000s models with limited cold-weather capability. Modern inverter-driven heat pumps with vapour injection technology are dramatically better. The myth also persists through fossil fuel industry lobbying.

How does efficiency change in winter?

A heat pump achieving COP 4.0 at 7°C typically delivers COP 2.5-3.0 at 0°C and COP 2.0-2.5 at -10°C. Even at the lower end, it produces more than double the heat of a direct electric heater and outperforms gas boilers.

Do Scandinavian countries use heat pumps?

Extensively. Norway has over 1.3 million installations, Sweden over 700,000, Finland over 1 million. These countries experience -20°C to -30°C winters routinely — far colder than anything in the UK.

Should I get a backup boiler for cold snaps?

No. A properly sized modern heat pump handles UK winters without backup. The coldest UK temperatures are well within operating range. A backup system adds unnecessary cost and complexity.