What is a heat pump?

A heat pump is a system that works by drawing heat from an external source like the air, ground or water, and raising it to a useful temperature. The heat is then used for space heating and other applications.

Heat pumps generally operate at lower flow temperatures than traditional heating systems such as boilers, typically in the range of 35-55°C rather than the 60-80°C commonly used by conventional radiator systems. For this reason, they are most efficient when paired with systems that require a lower output such as underfloor heating, which typically operates at around 30-40°C while still maintaining comfortable room conditions. With careful system design and where necessary correctly sized radiators, standard heat pumps can also work well with existing radiator systems.

For retrofit situations where replacing radiators would be disruptive or costly, high temperature heat pump models are available that can reach 70-80°C. They are comparable to a conventional boiler and can often work with a property’s existing heating infrastructure without modification. High temperature variants are available across air source, ground source and water source systems.

Electricity is required to convert the heat that is harvested into usable heating. This means that it is important that the harvested heat is more than the energy used to convert the heat, in order for this system to be efficient. The key measure of a heat pump’s efficiency is its Coefficient of Performance (COP). It is the ratio of heat energy delivered to electrical energy consumed. For example, a COP of 3 means that for every 1 kWh of electricity used, 3 kWh of heat is produced. There is also the Seasonal Coefficient of Performance (SCOP), which averages performance across an entire heating season and gives a more realistic picture of annual efficiency.

Because heat pumps produce significantly more heat than the electricity they consume, running costs depend on your electricity tariff. The efficiency advantage of heat pumps helps to offset the fact that electricity costs more per unit than gas, but actual savings will depend on how well the system is designed and operated.

While heat pumps perform best in well insulated buildings, advances in system design and high temperature models mean they can now be viable in a much wider range of properties, including older homes. Heat pumps also generally outlast conventional boilers, with most systems lasting 20 years or more with proper maintenance. As the UK electricity grid continues to decarbonise, the carbon benefit of heat pumps is expected to improve further over time.

Planning and environmental requirements vary depending on the type of heat pump and your location. Therefore make sure you always confirm with your local authority or a specialist before proceeding.

Types of heat pump

The types of heat pumps available are ground source, air source and water source heat pumps – some being more effective and more utilised than others. There can also be hybrid heat pumps that are used in combination with a traditional boiler. Let’s take a look at each one.

Air Source Heat Pumps (ASHP)

Air source heat pumps work by absorbing heat from the outside air (even when it is cold) and upgrading it using a heat exchanger to be used as space heating or hot water. Because they extract heat from the ambient air, they require no groundworks, making them the most straightforward and cost-effective heat pump to install.

There are two types of air source heat pumps:

• Air to air
• Air to water

The air to air system produces warm air that is then circulated directly through the building. Air to water systems transfer the heat to a thermal store which is then connected to the underfloor heating system, heat water for radiators or hot water tanks. The units look similar to an air conditioning unit.

Modern ASHPs have improved significantly in recent years. Current-generation units are capable of operating efficiently down to -15°C or below, with some models reaching -25°C. This is well beyond anything the UK typically experiences even in the coldest winters.

The air source heat pump works best with underfloor heating or correctly sized radiators. Whether existing radiators need upsizing depends on the specific property, its heat loss, and the model selected. A proper heat loss calculation carried out by a qualified designer will determine this.

For domestic hot water, ASHPs typically work via a hot water cylinder and may require a supplementary immersion heater to periodically raise water temperatures to the 60°C recommended for hygiene and Legionella prevention.

In terms of location, ASHPs are well suited to urban and suburban settings, as they don’t require any land for ground works. However, be mindful of both the noise and visual impact of the external unit.

Advantages of Air Source Heat Pumps

• Low running costs
• Long lifespan
• Renewable heating system
• Lower carbon emissions
• No fuel delivery required
• Low maintenance
• Provides both heating and cooling
• Suitable for a wide range of homes

Disadvantages of Air Source Heat Pumps

• Generally the least efficient of the three heat pump types
• Visual impact
• Noise impact depending on unit
• May require radiator upsizing in some retrofit situations (a heat loss calculation will determine this)
• May need hot water cylinder space

Ground Source Heat Pumps (GSHP)

Now, we will explore ground source heat pumps.

The ground source heat pump system extracts heat from the ground using plastic piping that is buried in the ground. The pipes are filled with a fluid – a mixture of water and antifreeze, known as heat transfer fluid (HTF) or brine that flows into the heat pump. This absorbs the heat from the ground at low temperatures and passes it through a refrigerant which compresses it and raises its temperature. A heat exchanger then extracts the heat and transfers it to be used in your home, via radiators or underfloor heating. The fluid is then discharged back into the underground loop for the process to start again. This is known as a closed loop system.

Since the ground stays at a relatively constant temperature, this system can be used all through the year. GSHPs deliver stable, consistent performance in all seasons which is a key advantage over air source systems.

There are also open loop GSHP systems wherein groundwater is drawn from an aquifer, passed through the heat pump to extract heat, and then discharged to a separate borehole. This system may require environmental permits.

Both ground source heat pumps and water source heat pumps can have closed and open loop systems.

The ground loops can be configured in a few different ways, the selection of which is dependent on land available, cost, ground conditions, and other variables.

Horizontal loops are installed horizontally in trenches, at a depth of around 1.5 to 2 metres deep. The actual depth will depend on soil conditions and the design of the system. These horizontal loops will require a larger surface area than the vertical loops. We can expect to see anywhere between 100m-400m of piping per individual loop. The area you need is generally 2-3 times the floor area of your building. The precise piping and ground loop requirements should be determined by a qualified ground source heat pump installer or specialist consultant, as they will vary significantly by site.

On larger scale projects, a horizontal loop system is well suited to be installed below a car park or perhaps play area for obvious reasons – large surface area with plenty of space for the system, with minimal underground interruptions or services.

It is important that there is proper road access for machinery to dig the trenches and no obstructions like trees nearby due to their roots causing issues.

Vertical loop systems require far less space than the horizontal system, by inserting the pipework into vertical boreholes at varying depth. These typically range from around 50 m to over 150 m deep. Soil conditions and system design will have an impact on the vertical loop depth. This system will also be more expensive due to machinery and specialist ground survey costs.

Finally, the slinky coil system reduces the volume required by overlapping the coils of piping in either a horizontal or vertical fashion. This system can reduce land area requirements and depth requirements for a vertical system. Do note that the overlapping layout in a slinky coil system will extract heat from a smaller volume of ground than a straight pipe system. This tight spacing causes the ground to cool down much faster which directly lowers the heating efficiency of the system. This is why proper sizing and spacing is essential for this system.

For domestic hot water, standard GSHPs typically work via a hot water cylinder. Modern high temperature models can reach the 60°C recommended for Legionella prevention directly, though many systems incorporate a supplementary immersion heater to periodically raise water temperatures to this level as needed.

In terms of location, suburban or rural locations tend to be the most suitable for GSHP. Urban areas often prohibit use of GSHP due to lack of land area, but also because of underground services competing for space. You will also need to leave out some space on the inside of your building for the indoor heat pump unit containing the hot water cylinder.

Advantages of Ground Source Heat Pumps

• Low running costs
• Long lifespan
• Low maintenance
• Renewable heating system
• Provides both heating and cooling
• No visual impact
• Lower carbon emissions

Disadvantages of Ground Source Heat Pumps

• Requires large amount of space
• High installation costs
• High disruption during installation due to groundworks
• Pipework can limit landscaping opportunities
• Not well suited to urban locations due to land requirements and underground services
• May need hot water cylinder space

Water Source Heat Pumps (WSHP)

Moving on to water source heat pumps.

This heat pump system is similar to the ground source heat pump, but the pipes are immersed in a body of water instead of the ground. The water sources can be lakes, streams, rivers, aquifers, mine water and even seawater.

These heat pumps can be highly efficient, as water bodies tend to maintain relatively stable temperatures and have a high thermal capacity. This option also requires large amounts of open unshaded space. They may not be suitable for all locations and are typically the most complex and expensive to design and install. However, where a suitable water source is available, they can offer excellent efficiency and performance even in cold climates.

Like we mentioned before, water source heat pumps can employ both closed and open loop piping systems. In a closed loop system, a sealed pipe carrying thermal transfer fluid is submerged in the water body to absorb heat.

In the open loop system, water is drawn directly from a natural source such as a borehole near a river then passed through a heat exchanger where heat is extracted before the cooler water is returned to a separate borehole or discharge point. Because no antifreeze is used, there is a risk of freezing in the system which is why careful installation is essential. Clogging from debris or sediment can also be an issue. Open loop systems require appropriate environmental permits and compliance with regulations to do with water abstraction and discharge.

Advantages of Water Source Heat Pumps

• Efficient due to stable water temperatures
• Works well even in cold climates
• Provides both heating and cooling
• Long lifespan
• Low running costs
• Lower carbon emissions

Disadvantages of Water Source Heat Pumps

• High installation costs
• Requires access to a suitable and proximate water source
• Complex design and installation
• Environmental regulations may apply
• Maintenance of water loop is required
• Performance depends on water quality and availability
• Not suitable for all locations

Hybrid Heat Pumps

Now let’s briefly cover hybrid heat pump systems. These systems combine a heat pump with a conventional heating system (usually a gas or oil boiler), forming one integrated system.

The heat pump handles the bulk of the heating load under normal conditions, with the boiler stepping in during peak demand or very cold weather. Smart controls manage the switchover automatically. This makes hybrid systems a practical transitional solution, particularly for older homes and retrofit projects.

However, because the system still relies on fossil fuels, the carbon savings are more limited than a fully renewable heat pump system. Moreover, hybrid installations may not be eligible for some government grants.

Advantages of Hybrid Heat Pumps

• Reliable back up system
• Long lifespan
• Lower upfront disruption (can keep existing boiler)
• Can reduce energy bills compared to boiler only systems
• Good transitional solution for older homes
• Improved performance during cold weather

Disadvantages of Hybrid Heat Pumps

• Higher installation cost than single systems
• More complex system with the combination of two technologies
• Higher maintenance requirements
• Still relies on fossil fuels
• Lower carbon savings than fully renewable systems