Air source heat pump: how it works and what it heats in UK homes

Part of our guide on characteristics of green homes

What is an air source heat pump

An air source heat pump (ASHP) captures heat from the outside air and transfers it indoors to heat your home or supply hot water. It works similarly to a refrigerator running in reverse but is designed to provide warmth instead of cooling.

Typically, an ASHP has an external unit containing a fan and heat exchanger, which draws in air and extracts heat from it, even when outside temperatures are as low as -15°C. This heat is then compressed and transferred inside to warm your living space or water.

In UK homes, an ASHP usually replaces traditional gas or electric boilers, offering a renewable alternative that reduces carbon emissions and energy bills when installed and sized correctly.

How an air source heat pump extracts heat from outside air

The ASHP’s outdoor unit uses a refrigerant fluid that evaporates at very low temperatures. As the fan pulls in outside air, the refrigerant absorbs heat, changing from liquid to gas. For the detail, see our notes on what is modular and prefabricated housing.

A compressor then squeezes this gas, raising its temperature substantially—often above 50°C—before it moves indoors. Inside, the heat passes through a heat exchanger, warming water or air for your heating system.

Despite cold UK winters, most modern ASHPs can extract useful heat down to around -15°C. Below this, efficiency drops, and supplementary heating might be needed. The heat pump’s performance depends heavily on outdoor temperature, humidity, and unit quality.

The coefficient of performance (COP) for an ASHP varies with outdoor temperature; for example, at 7°C, a typical ASHP might deliver a COP of 3.5, meaning it produces 3.5 units of heat for every unit of electricity consumed. However, as temperatures drop closer to -15°C, the COP can fall to around 2.0 or less, indicating reduced efficiency. Therefore, when sizing an ASHP, installers must consider local climate data to ensure the system can meet heating demands even on colder days. We go through smart thermostat setup guide step by step elsewhere on the site.

A worked example: if an ASHP with a COP of 3.5 consumes 1 kWh of electricity, it delivers 3.5 kWh of heat to the home. If the outside temperature falls and the COP drops to 2.0, the same 1 kWh of electricity now produces only 2 kWh of heat, which may necessitate supplementary heating to maintain comfort levels. Monitoring this performance helps homeowners anticipate changes in running costs during winter months.

What does an air source heat pump heat in UK homes

ASHPs heat indoor spaces primarily by warming water that flows through radiators or underfloor heating systems. The water temperature typically reaches between 35°C and 55°C, which means radiators must be larger or underfloor heating used to achieve comfortable warmth.

They also supply domestic hot water for taps and showers, though producing water at 60°C or above often requires extra energy input or a heat pump with a built-in boost heater. The other half of this decision is efficient home heating methods.

Some systems use the warmed air directly by blowing it through vents in the living areas, but water-based delivery is more common in the UK. The system’s design must match your home’s heating needs and existing infrastructure to avoid inefficiency or discomfort.

In homes with existing radiator systems designed for high-temperature water (around 70-80°C), replacing a boiler with an ASHP may require upgrading to larger radiators or switching to underfloor heating to compensate for the lower water temperatures. A typical ASHP heats water to approximately 35-55°C, which is sufficient for well-insulated homes but may struggle in poorly insulated properties.

To verify system effectiveness, installers often perform a heat loss calculation for the home to determine the required heating output. For instance, a well-insulated 100 m² home might need a heat output of 5 kW; selecting an ASHP that can deliver this at the lower water temperature ensures comfort. If the system cannot meet this need, homeowners may experience cold spots or rely more on electric backup heaters, increasing energy costs. People in this spot often ask about solar thermal explained as well.

Differences between air source and ground source heat pumps

Ground source heat pumps (GSHPs) extract heat from the soil or groundwater, where temperatures remain more constant, usually around 10-12°C year-round. This stability often results in higher efficiency compared to ASHPs but requires significant ground works.

ASHPs are easier and cheaper to install because they do not need underground loops or boreholes; instead, the external unit is mounted outside on a wall or on the ground. However, their efficiency drops more in cold weather than GSHPs.

GSHP systems typically provide higher heat output at lower electricity use but involve disruption during installation and higher upfront costs. Choosing depends on your property’s space, soil type, and budget. For the detail, see our notes on u-value in construction.

How air source heat pumps differ from air conditioners

Though ASHPs and air conditioners share similar components like compressors and refrigerants, their core purposes differ. Air conditioners remove heat from inside a building to cool it, releasing that heat outside.

ASHPs move heat in the opposite direction during winter, taking heat from outside air and delivering it indoors to warm rooms. Many modern ASHPs can reverse this process, providing cooling in summer similarly to an air conditioner.

The key difference lies in design optimisation: ASHPs focus on heating efficiency at low outside temperatures, while air conditioners are optimised for cooling performance when outdoor temperatures are higher. For the detail, see our notes on what is a fuel cell boiler.

What grants and funding schemes support air source heat pumps in the UK

UK government schemes like the Boiler Upgrade Scheme (BUS) and ECO4 offer grants covering up to £5,000 for air source heat pump installation to eligible homeowners and landlords.

The Boiler Upgrade Scheme requires the heat pump to meet efficiency standards, typically a seasonal coefficient of performance (SCOP) above 3.0, ensuring systems deliver real energy savings.

Additional funding might be available through local councils under the Warm Homes Plan, focusing on low-income households. Eligibility conditions depend on factors like income, property EPC rating, and current heating system.

Applying for these grants involves getting a professional assessment, ensuring your property is suitable, and using certified installation companies.

The Boiler Upgrade Scheme also requires installations to be completed by Microgeneration Certification Scheme (MCS) certified installers, ensuring quality and compliance with industry standards. This certification helps maintain performance and reliability while protecting consumers.

Post-installation, homeowners may need to submit evidence such as installation certificates and product details to claim grants. A common check involves verifying the heat pump’s seasonal performance factor (SPF) through commissioning reports, confirming that the system meets promised efficiency levels.

Furthermore, some local authorities offer additional incentives or reduced business rates for properties installing renewable heating technologies like ASHPs. These schemes vary widely, and prospective buyers should consult local council websites or energy advisors to explore all available financial support options.

• Boiler Upgrade Scheme: up to £5,000 towards ASHP installation
• ECO4 scheme: helps low-income or vulnerable households
• Grants require ASHPs to meet efficiency and emission standards
• Professional assessment needed before installation

Questions people still ask

I have installed and maintained air source heat pumps in UK homes and understand their practical strengths and limits firsthand.