How Does an Air Source Heat Pump Work

Heating is one of the largest sources of energy use in UK homes, and with rising energy costs and pressure to reduce carbon emissions, many people are looking at alternatives to traditional gas or oil boilers. One of the most widely discussed solutions is the air source heat pump. Although the name is increasingly familiar, the way these systems work is not always well understood.

This article explains how an air source heat pump operates, why it is considered efficient, and what makes it different from other heating systems. By breaking down the process in simple terms, homeowners, landlords and developers can gain a clear picture of how these systems function and whether they might be suitable for their property.

What is an Air Source Heat Pump?

An air source heat pump is a low carbon heating system that extracts heat from the outside air and transfers it into a building to provide space heating and, in many cases, hot water. Even when the outdoor temperature is close to freezing, the system can still capture and concentrate heat from the air, which makes it a practical option for the UK climate.

Air source heat pumps are electrically powered, but instead of generating heat directly, they move existing heat from one place to another. This is why they are often described as being similar to a refrigerator in reverse. A fridge removes heat from inside its cabinet and releases it into the room, while an air source heat pump takes heat from outside air and releases it into the home.

The Science Behind Heat Pumps

The principle of an air source heat pump is based on the refrigeration cycle. This process relies on a refrigerant, which is a fluid that can easily change state between liquid and gas. The refrigerant absorbs heat from the outside air, even at low temperatures, and then transfers this heat into the system inside the home.

The refrigerant passes through a closed loop of components, including an evaporator, compressor, condenser and expansion valve. These parts work together to absorb heat from the air, increase its temperature and release it at a usable level indoors. The system can then distribute the heat through radiators, underfloor heating or a hot water cylinder, depending on the property’s setup.

Step by Step: How Heat is Transferred

When the outdoor unit of an air source heat pump is running, air is drawn in by a fan and passed over coils containing the refrigerant. The refrigerant absorbs heat from the air and evaporates into a gas.

This gas is then compressed by the compressor, which raises its temperature significantly. Once compressed, the hot refrigerant gas flows into the condenser, where it releases its stored heat into the property’s heating system. This heat can be transferred into water that circulates through radiators or underfloor pipes, or into a hot water cylinder for domestic use.

After releasing its heat, the refrigerant cools and condenses back into a liquid. It then passes through the expansion valve, which lowers its pressure and temperature before returning it to the evaporator to absorb more heat from the outside air. This cycle repeats continuously whenever the system is in use.

How Efficient are Air Source Heat Pumps?

One of the main attractions of an air source heat pump is its efficiency. Unlike a gas boiler, which burns fuel to create heat, a heat pump uses electricity to move existing heat. This allows it to produce more energy in the form of heat than it consumes in electricity.

For example, a well-installed air source heat pump might have a coefficient of performance (COP) of around 3. This means that for every unit of electricity it uses, it can generate around three units of heat. The exact efficiency depends on outdoor temperatures, the design of the heating system and the quality of installation, but in general heat pumps are significantly more efficient than direct electric heating.

Can Heat Pumps Work in Cold Weather?

A common concern among homeowners is whether air source heat pumps can operate effectively in colder weather. In reality, these systems are designed to work in low temperatures and can still extract heat from the air even when it is below zero outside.

Although efficiency may drop slightly in freezing conditions, modern heat pumps are built to cope with the UK’s winter climate. Many systems also include a backup immersion heater for hot water, ensuring a reliable supply even during particularly cold periods.

Integration with UK Homes

For a heat pump to perform at its best, the property itself needs to be suitable. Homes with good levels of insulation and draught proofing will benefit the most, as heat pumps deliver lower flow temperatures than gas boilers. This means radiators may feel cooler to the touch, but the system is designed to run for longer periods, providing a steady, consistent warmth throughout the home.

Heat pumps work especially well with underfloor heating, as this system operates at lower temperatures and provides even distribution of heat. That said, many existing homes can be retrofitted with larger radiators or improved insulation to make them compatible with heat pump technology.

Environmental Benefits

The environmental case for air source heat pumps is one of the strongest reasons for their adoption. By replacing a fossil fuel boiler, a heat pump can significantly reduce the carbon footprint of a property. The system itself produces no direct emissions, and as the UK electricity grid continues to add more renewable energy sources, the overall environmental impact of heat pumps will decrease further.

This makes them a key technology in the government’s strategy to reduce carbon emissions from heating and to move towards net zero targets. For homeowners and landlords, installing a heat pump can also improve the property’s energy performance certificate (EPC) rating, which is increasingly important in the housing market.

Maintenance and Reliability

Air source heat pumps are generally reliable and require less maintenance than traditional boilers. However, regular servicing is important to ensure efficiency and longevity. This usually involves checking refrigerant levels, cleaning filters and inspecting the outdoor unit to ensure there is no obstruction to airflow.

A professionally installed and well-maintained heat pump can last 15 years or more, making it a long-term investment in both comfort and sustainability.

Costs and Support

The cost of installing an air source heat pump can be higher than replacing a boiler, but there are schemes designed to help with affordability. Government initiatives such as the Boiler Upgrade Scheme in England and Wales provide grants to support the installation of low carbon heating. Over time, the running costs of a heat pump can be lower than gas or oil heating, particularly in well-insulated properties.

Conclusion

Air source heat pumps provide an efficient, low carbon way to heat homes and water. They work by capturing heat from the outside air, transferring it through a refrigeration cycle and releasing it into a property’s heating system. Even in cold weather, they are capable of delivering consistent warmth while using far less energy than direct electric heating.

For households and landlords looking to reduce running costs and environmental impact, an air source heat pump is a viable and increasingly popular alternative to traditional heating. With the right property setup and professional installation, it can provide reliable, sustainable comfort for many years.