The principle: heat flows downhill, so push it uphill
Heat always flows from something warmer to something colder. A cup of tea cools down; a cold drink warms up. To get heat to flow the other way, from cold outside air into a warm house, you need to do work. A heat pump does that work with a refrigerant, a fluid chosen because it boils at very low temperatures and changes between liquid and vapour easily.
Two facts about fluids make it possible. When a liquid boils, it absorbs a lot of heat without getting warmer. And when you compress a vapour, its temperature rises. A heat pump uses the first to collect heat outside and the second to lift that heat to a temperature that can warm a home.
The cycle, one step at a time
- Take heat. Very cold refrigerant in the outdoor coil boils as outside air warms it, even on a frosty day.
- Squeeze. The compressor, the part that uses electricity, compresses the vapour, which makes it hotter.
- Give heat. The hot vapour passes its heat to the water for radiators and taps, and condenses back to liquid.
- Release. The expansion valve drops the pressure, the liquid turns very cold again, and the loop repeats.
1. Take heat from outside
In an air source heat pump, a fan draws outside air across a coil called the evaporator. The refrigerant inside the coil is colder than the air, even on a frosty day, so heat flows from the air into the refrigerant and it boils into a vapour. In a ground source heat pump the same thing happens, but the heat comes from fluid circulating through pipes buried in the ground.
2. Squeeze it
The vapour goes to the compressor, the heart of the machine and the part that uses most of the electricity. Compressing the vapour packs the same heat into a smaller volume and raises its temperature well above that of the outside air. This is the "pump" in heat pump: it lifts heat from a low temperature to a useful one.
3. Give the heat to the home
The hot vapour flows through a second heat exchanger, the condenser. On the other side is the water that circulates through the radiators or underfloor pipes, or that heats the hot water cylinder. Heat passes from the refrigerant to the water, and as it gives up its heat the refrigerant condenses back into a liquid.
4. Release the pressure
The warm, high-pressure liquid passes through an expansion valve, which drops the pressure sharply. That makes the refrigerant very cold again, colder than the outside air, ready to go back to the evaporator and collect more heat. The loop runs continuously while the system is heating.
Why it delivers more heat than the electricity it uses
The electricity does not become the heat, as it would in a fan heater. It runs the compressor, fans and pumps that move heat which was already outside. So the heat delivered into the home is the heat collected from outside plus the energy the compressor adds. The government's explainer puts the ratio at around 3 units of heat for every unit of electricity; a field trial of British homes, the Electrification of Heat demonstration, recorded an interim median of 2.80 for air source heat pumps over a season.
That ratio is not fixed. It is highest when the outside air is mild and the water sent to the radiators is cool, and lowest when it is very cold outside and the radiators need very hot water. Almost every design decision about a heat pump is really about keeping that ratio high.
What sets the efficiency
- The temperature lift. The bigger the gap between the heat source and the water temperature the home needs, the harder the compressor works. That is why flow temperature matters so much.
- The heat source. The ground changes temperature less over the year than the air, which helps a ground source system on cold days. Air is easier and cheaper to reach.
- The size of the system. A heat pump sized to the home's heat loss runs long and steady. One that is too big cycles on and off, which wastes energy.
- The controls. Weather compensation lowers the water temperature on mild days; see controls.
Hot water
An air-to-water or ground source heat pump also heats the water for taps and showers, usually through a well-insulated cylinder. Hot water needs a higher temperature than space heating, so the heat pump is less efficient while it does it. The hot water guide explains cylinders and reheating.
What a heat pump needs in the home
The government's explainer lists the practical points plainly: you need space for the heat pump and for a water cylinder, and some radiators may need to be changed for larger ones. It also quotes Energy Systems Catapult saying heat pumps can operate efficiently at moderate and high flow temperatures, which matters for homes that cannot change every radiator. Whether a particular home suits one is covered in is my home suitable?
None of this needs to be understood to use a heat pump day to day. But it explains the advice you will hear: keep the water temperature low, let it run steadily, and size it properly. Each of those keeps the lift small and the ratio high.
The same cycle underlies every design described in types of heat pump. How well it performs in a particular house is the subject of is my home suitable?, and how its output is measured is explained in COP, SCOP and SPF.