How do heat pumps work?

Heat pumps transfer existing heat from one location to another, rather than generating it through combustion, making them a highly efficient way to warm your home. This technology is key to decarbonising home heating in the UK and reducing reliance on fossil fuels.

Understanding the basics: what is a heat pump?

Unlike traditional boilers that burn fuel to create heat, a heat pump simply moves warmth from one place to another. This fundamental difference is what makes them so efficient and a powerful tool for energy independence.

Heat transfer, not generation

Heat pumps operate on the principle of heat transfer. They extract thermal energy from ambient sources like the air, ground, or water, even at low temperatures, and then release that heat into your home. Think of it like a refrigerator, but in reverse: instead of moving heat out of a cold space, it moves heat into a warm one. This process requires electricity to run the system, but the amount of heat delivered is significantly greater than the electrical energy consumed.

The refrigerant cycle explained

The core of a heat pump's operation is a continuous cycle involving a special fluid called a refrigerant. This refrigerant changes between liquid and gas states, absorbing and releasing heat as it circulates through the system. This cycle allows the heat pump to capture low-grade heat from outside and upgrade it to a higher temperature suitable for heating your home and hot water.

The core components of a heat pump system

A heat pump system comprises four main components that work together to facilitate the refrigerant cycle and transfer heat efficiently.

Evaporator: absorbing heat

The evaporator is where the magic of heat absorption begins. Located outside your home (for air source heat pumps) or underground (for ground source), the liquid refrigerant passes through it. Even in cold conditions, the ambient air, ground, or water contains enough thermal energy to cause the refrigerant to evaporate and turn into a low-pressure gas. The refrigerant has an extremely low boiling point, meaning it can absorb heat even when outside temperatures are well below freezing, such as -20°C.

Compressor: raising temperature

Once the refrigerant has turned into a gas, it moves to the compressor. This component, powered by electricity, increases the pressure of the gaseous refrigerant. As the pressure rises, so does the temperature of the refrigerant, upgrading the captured low-grade heat to a level suitable for home heating.

Condenser: releasing heat

The hot, high-pressure refrigerant gas then flows into the condenser, typically located inside your home. Here, the heat is transferred from the refrigerant to your home's heating system (radiators or underfloor heating) and hot water tank. As it releases its heat, the refrigerant cools down and condenses back into a high-pressure liquid.

Expansion valve: cooling down

Finally, the high-pressure liquid refrigerant passes through an expansion valve. This valve reduces the pressure of the refrigerant, causing it to cool down significantly. Now a low-pressure, cool liquid, it returns to the evaporator to begin the cycle again, ready to absorb more heat from the environment.

Different types of heat pumps for UK homes

In the UK, homeowners typically consider a few main types of heat pumps, each suited to different property types and outdoor spaces.

Air source heat pumps

Air source heat pumps (ASHPs) are the most common type in the UK and are suitable for most homes. They extract heat from the outside air, even in cold winter temperatures. ASHPs typically consist of an outdoor unit that resembles an air conditioning unit and an indoor unit that connects to your heating system. They can be air-to-water, heating water for radiators and hot taps, or air-to-air, which distribute warm air directly.

Ground source heat pumps

Ground source heat pumps (GSHPs) utilise the more stable temperature of the earth to extract heat. They involve a network of pipes buried underground, either horizontally in trenches or vertically in boreholes. GSHPs are generally more efficient than air source systems because ground temperatures are more consistent than air temperatures, but they are also more expensive to install due to the excavation required.

Water source heat pumps

Water source heat pumps (WSHPs) are less common for residential properties but can be highly efficient if your home is near a suitable body of water, such as a lake, river, or large pond. These systems extract heat from the water and transfer it to your home's heating system.

Heat pumps in the UK climate: performance and efficiency

A common concern for UK homeowners is how heat pumps perform in colder weather. Modern heat pumps are designed to operate effectively in the UK's climate, offering impressive efficiency.

Operating in cold weather

Heat pumps are well-suited to the UK's climate and can extract heat from the air, ground, or water even in freezing conditions. While their efficiency may decrease slightly at extremely low temperatures, modern air source heat pumps can operate effectively down to around -15°C, with some models even functioning at -20°C to -25°C. Countries with much colder climates than the UK, such as Norway and Sweden, widely use heat pumps for heating.

Coefficient of performance (CoP)

The efficiency of a heat pump is measured by its Coefficient of Performance (CoP). This is the ratio of the heat energy produced to the electrical energy consumed. Heat pumps typically have a CoP of 3 or more, meaning they produce 3 or more units of heat for every unit of electricity they use. This makes them significantly more efficient than traditional boilers.

Factors affecting efficiency

Several factors influence a heat pump's efficiency. Proper home insulation is crucial; well-insulated homes retain heat better, allowing the heat pump to work less to maintain a comfortable temperature. The size and type of heat emitters (radiators or underfloor heating) also play a role, as heat pumps generally work best by providing consistent, lower-temperature heat over longer periods. Running a heat pump at lower flow temperatures (typically 35-55°C for radiators or underfloor heating) significantly improves its efficiency compared to higher temperatures.

Benefits and considerations for homeowners

Deciding whether a heat pump is right for your home involves weighing its environmental benefits and potential cost savings against installation requirements and initial investment.

Environmental impact and running costs

Heat pumps offer a significant environmental advantage by reducing carbon emissions, as they do not burn fossil fuels directly. When powered by renewable electricity, their carbon footprint is very low. While electricity is generally more expensive per unit than gas in the UK, the high efficiency of heat pumps can lead to comparable or even lower running costs than a gas boiler, especially in well-insulated homes.

Installation requirements and suitability

Heat pumps are not a one-size-fits-all solution, and their suitability depends on your property. While modern heat pumps can be installed in homes of all ages and types, including older properties, good insulation is important for optimal performance. Some older homes may require upgrades to insulation or radiators to ensure the system operates efficiently. Space is also a consideration, as air source heat pumps require an outdoor unit, and ground source systems need significant land for pipework.

Government support and grants

To encourage the adoption of low-carbon heating, the UK government offers financial incentives. The Boiler Upgrade Scheme (BUS) provides grants to eligible homeowners in England and Wales towards the cost of installing a heat pump. Since 21 July 2026, the standard grant is £7,500 for air source or ground source heat pumps. For qualifying off-gas grid homes currently heated by oil or LPG, an enhanced grant of £9,000 has been available since 21 July 2026 until 31 March 2027. To be eligible for these grants, installations must be carried out by an MCS (Microgeneration Certification Scheme) certified installer, who applies for the grant on the homeowner's behalf.