Why Heat Pumps Work Below Freezing, and What the Coefficient of Performance Means

The bit people find hard to believe

The usual objection to heat pumps is simple: how can something extract heat from air that feels freezing cold? The answer is that “cold” is relative. Air only becomes genuinely free of heat at absolute zero, around minus 273 degrees Celsius. Anything warmer than that, including a frosty morning in the UK, still contains a substantial amount of thermal energy. A heat pump does not need the air to feel warm to us; it just needs the air to be warmer than the refrigerant circulating inside the machine.

The refrigeration cycle, run in reverse

A heat pump works on the same principle as a fridge, just pointed the other way. Inside the unit is a refrigerant, a fluid engineered to boil and condense at very low temperatures. The cycle has four basic stages:

• The refrigerant, kept at low pressure, passes through an outdoor coil called the evaporator. Because its boiling point is so low, even chilly outside air is enough to make it evaporate into a gas, absorbing heat as it does so.
• A compressor squeezes that gas, raising both its pressure and its temperature significantly. This step uses electricity and is the main energy cost of the whole system.
• The now-hot, high-pressure gas flows through a second coil, the condenser, usually connected to the home’s heating system. It condenses back into a liquid, releasing the heat it is carrying into the house.
• An expansion valve drops the pressure again, cooling the liquid refrigerant back down so it can re-enter the evaporator and repeat the cycle.

The trick is that compressing a gas raises its temperature well above the temperature of the air it was drawn from. So even air a couple of degrees above freezing can, after compression, produce refrigerant hot enough to heat radiators or underfloor pipes.

Why below-freezing air still works

As long as the refrigerant’s boiling point is lower than the outside air temperature, the evaporator can still pull in heat. Modern refrigerants are chosen specifically so that this holds true well into negative figures on the Celsius scale, which is why air source heat pumps are used routinely in much colder climates than the UK’s, including Scandinavia and Canada.

There is a practical complication, though. Very cold, humid air can cause frost to build up on the outdoor evaporator coil, which blocks airflow and reduces performance. Heat pumps handle this with a defrost cycle, briefly reversing the process to melt the ice, then switching back to heating. This is a normal, designed-in part of operation, not a fault.

What the coefficient of performance actually measures

The coefficient of performance, or COP, is simply the ratio of heat energy delivered into the home to the electrical energy used to run the compressor and pumps. If a system has a COP of three, it means three units of heat come out for every one unit of electricity put in. That might look like it breaks the rules of energy conservation, but it does not: the heat pump is not creating energy, it is moving existing heat from outside to inside, and the electricity is only the energy needed to drive that movement, not the heat itself.

COP is not fixed. It depends heavily on the temperature difference the pump has to work across. The smaller the gap between the outside air and the temperature the heating system needs to reach, the less work the compressor has to do, and the higher the COP. This is why heat pumps paired with underfloor heating or oversized radiators, which need lower flow temperatures than a traditional gas-fired system, tend to run more efficiently than those forced to produce very hot water for small radiators. It also explains why COP drops on the coldest days: the temperature difference is larger, so the compressor works harder for each unit of heat delivered. Crucially, COP on a well-designed system should still stay comfortably above one even in freezing conditions, meaning it remains more efficient than a heater that simply converts electricity directly into heat.

Because COP varies throughout the year, manufacturers and installers also quote a seasonal figure, often called SCOP, which averages performance across a typical year of temperature swings. This seasonal figure is a far more useful guide to real running costs than a single COP measured in ideal lab conditions, since it reflects how the system behaves through both mild autumn days and hard winter cold snaps.

What this means for a household

The practical takeaways are that heat pump performance depends on good sizing, adequate insulation, and a heating system designed to run at lower temperatures, not on some fixed efficiency number stamped on the box. Anyone considering one should ask for a proper heat loss survey and a SCOP estimate for their specific property, and check current guidance and any available support schemes through official sources before committing, since eligibility rules and incentive schemes change over time.

Further reading

For independent, up-to-date guidance on heat pumps, costs and support schemes, see MoneyHelper and the Energy Saving Trust, and consult Ofgem for information on energy markets and consumer protections.