The basic idea
A conventional electric heater turns electricity into heat directly. One unit of electrical energy in gives, at best, one unit of heat out. A gas boiler burns fuel and passes the heat on, losing some of it through the flue.
A heat pump does something different. It uses electricity to move heat from one place to another, typically from outside a building to inside. Because it moves existing heat rather than creating it, it can deliver more heat than the electrical energy it consumes. That sounds like cheating, but it does not break any physical law: the extra heat comes from the surroundings, not from nowhere.
Heat exists even when it feels cold
Outdoor air on a cold day still contains thermal energy. “Cold” is relative; heat only stops being available at absolute zero, which is far below any weather on Earth. A heat pump can extract some of that energy and release it at a higher temperature somewhere else. The ground and bodies of water are also used as heat sources in some systems.
The same principle runs in a refrigerator, in reverse. A fridge moves heat out of its cold interior and releases it from coils at the back. A heat pump does that on a larger scale, aimed at warming a building instead of cooling a cabinet. Many can also run backwards for cooling.
The refrigeration cycle
Most heat pumps use a closed loop containing a refrigerant, a fluid chosen because it boils and condenses at convenient temperatures. The loop has four main parts.
- Evaporator. The cold refrigerant passes through a heat exchanger that is exposed to outdoor air (or the ground). The refrigerant is colder than its surroundings, so heat flows into it. It absorbs enough energy to evaporate into a gas.
- Compressor. This is where the electricity is mostly used. The compressor squeezes the gas, raising its pressure and, as a result, its temperature. The gas is now hotter than the inside of the building.
- Condenser. The hot gas flows through a second heat exchanger inside the building. Because it is hotter than the room or the water in the heating system, it gives up heat and condenses back into a liquid. This is the heat that warms the home.
- Expansion valve. The liquid passes through a narrow restriction, which drops its pressure and cools it sharply. It is now cold enough to start the loop again.
The key insight is that pressure and temperature are linked. By changing the pressure of the refrigerant, the system makes it absorb heat in one place and release it in another.
Why this can be efficient
Engineers describe a heat pump’s performance with a coefficient of performance, often shortened to COP. It is the heat delivered divided by the electricity used. A COP of 3 means three units of heat for each unit of electricity, which some people describe as “300% efficient”. That wording can confuse, because it does not mean energy is created; it means two of the three units were moved from the surroundings.
Typical figures depend heavily on conditions, so treat any single number with caution. Several factors raise or lower performance:
- Temperature difference. The smaller the gap between the heat source and the temperature you want, the less work the compressor has to do. A heat pump heating a house from mild outdoor air is generally more efficient than one working in severe cold.
- Delivery temperature. Systems that heat the home with larger radiators, underfloor heating or warm air at lower temperatures tend to be more efficient than those that need very hot water.
- Quality of installation. Correct sizing, pipe runs and controls make a significant difference.
- Defrost cycles. In cold, humid weather, ice can form on the outdoor unit and must be melted, which uses energy.
Seasonal figures, which average performance across a year, are more useful than a single peak value. This is a good example of reading efficiency claims critically: ask under what conditions a number was measured.
Types of heat pump
- Air-source. These take heat from outdoor air. They are the most common and generally cheapest to install.
- Ground-source. These use pipes buried in the ground or in boreholes. The ground temperature is steadier than the air, which can help efficiency, but installation is more involved and costly.
- Water-source. These draw from a lake, river or other water body where conditions allow.
- Air-to-air versus air-to-water. Some deliver warm air directly into rooms; others heat water for radiators, underfloor circuits and hot water tanks.
Practical considerations
A heat pump is not a drop-in replacement in every home. Insulation, existing radiators, available outdoor space, noise rules and the local electricity price relative to other fuels all affect whether it makes sense. A well-insulated home with suitable emitters often gets better results than a draughty one.
Because heat pumps are a large electrical load, they also interact with the rest of the energy system. They add to winter demand on the power grid, and they pair naturally with thermal storage, such as a hot water cylinder. Home energy monitoring can show how much electricity a unit really uses over a season, which is the best way to check performance in your own building.
Safety and professional work
Heat pumps contain refrigerant under pressure and are wired into the building’s electrical supply. In most places, handling refrigerants requires certification, and installation must follow local regulations. This is not a DIY job, and makers are better off learning the principle through reading, small demonstrations such as how a fridge works, or monitoring a professionally installed system. Always follow manufacturer guidance and local rules.