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Ground Source Heat Pumps Explained

a ground source heat pump being installed
(Image credit: Kismet Photography)

A ground source heat pump is a system that harnesses natural heat from the earth to provide low-carbon home heating as a replacement to a gas boiler. 

However, installing a ground source heat pump (GSHP) in your new build or in an existing property is no small commitment. The initial expenditure for a ground source heat pump system is high and the installation process is disruptive, however, the benefits of  GSHPs still make them a sound choice for the right property. 

In fact, ground source heat pumps are one of renewables touted for the future of home heating, along with air source heat pumps, with gas boiler installations to be banned in the UK from 2025 under the Future Homes Standard.

Get to know the technology better with our ultimate guide to ground source heat pumps, and see whether it’s the right choice for your property. 

(MORE: Get a quote for your heat pump)

What is a Ground Source Heat Pump?

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Ground source heat pumps use a series of buried pipes buried that extract the energy from the sun that warmed the ground. The heat pump then amplifies that energy into heat useful in the home. 

There are two main elements of a ground source heat pump system:

  • The ground array, which can be either a horizontal grid of pipes, which should be 1.2m below ground level, or two or three vertical boreholes, which are likely to be more than 70m deep. In either case the extent of the ground array will be determined by the size of the heat pump and the soil conditions 
  • The heat pump itself, which is installed in the house. Some of these include a hot water cylinder and can be the size of a large filing cabinet. The smallest pumps, such as those in the Kensa Heat Pump range, can be fitted in an under-sink kitchen cupboard, however, they're often located in dedicated plant rooms for easy access. 

Ground source heat pumps use electrical energy in a highly-efficient way to produce heat, significantly reducing heating running costs and CO2 emissions.  

While ground source heat pumps require electricity to operate, they can also be combined with renewable electricity sources such as solar PV panels. 

Pros and Cons of Ground Source Heat Pumps

Advantages of Ground Source Heat Pumps

  • Running cost is likely to be around 45% lower than for a gas boiler
  • If you sign-up to a renewable energy-only electricity supplier then CO2 emissions for the whole house, not just the heating system, will be effectively zero. You could also consider running a ground source heat pump along with solar PV panels to provide electricity for the heat pump. 
  • Once initial issues are dealt with you will have a heating system that is as reliable as a gas boiler and likely to last twice as long
  • There are grants available to offset the cost of ground source heat pumps 

Disadvantages of Ground Source Heat Pumps

  • The installation phase and start with the high capital cost – likely to be £8,000 to £10,000 for a typical 3 or 4 bedroom house 
  • The disturbance to the garden to install a horizontal or vertical ground array.
  • The cost and disruption of upgrading the radiators 

Types of Ground Source Heat Pumps

horizontal array ground source heat pump

An example of a horizontal ground array. (Image credit: Worcester Bosch)

A horizontal ground array will be either straight pipes or coiled (known as slinkies). There is no technical or efficiency difference between these and the installer (together with the manufacturer) will advise which is best for your situation. 

vertical or borehole array

An example of a vertical or borehole array. (Image credit: Worcester Bosch)

A vertical array will have boreholes drilled into the ground and connected across their tops.

The number and depth of boreholes will be dictated by the size of the heat pump and the geology. 

For instance, a 8kW heat pump is likely to need at least three boreholes around 70m deep. They do not need a large area of land and if the price is right, a vertical array is a good option. 

However, the price of boreholes can vary hugely across the country and can be prohibitively expensive.

(MORE: Installing a Ground Source Heat Pump)

a ground source heat pump submerged in water with pond mats

An example of a water source heat pump installation.  (Image credit: Kensa Heat Pumps)

Though far less common, you can also make use of bodies of water, such as lakes, rivers, ponds and streams, using a ground source heat pump system. Known as water source heat pumps, pipes are installed via pond mats submerged into the water.

Not only does this remove the need for disruptive digging, but the return energy to the heat pump is generally 5-6°C higher than standard collectors, improving the efficiency of the heat pump.

Heat pumps are generally installed close to an external wall to give easy access to the ground array pipes. There will also need to be space for a hot water cylinder but as they are very quiet in operation they generally sit comfortably in a utility room.

How Much Does a Ground Source Heat Pump Cost?

The cost of installing a typical ground source heat pump system in a 240m2 house will start from £12,000, depending on the type and quality of the heat pump system. 

The generally accepted ‘budget’ figure is £1,000 to £1,200 per kW capacity, varying with manufacturer and installation issues. 

A 200m2, four-bedroom house, built to Building Regulations standard, is likely to need an 8kW heat pump, costing around £6,000-£7,000, the balance being the installation cost which can vary significantly with the ground conditions. This compares to £1,000-£2,000 for a gas combi boiler. A biomass boiler, another renewable alternative, can cost anywhere from £11,000 to £25,000. 

(MORE: Ground Source Heat Pump Costs Explained)

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What are the Running Costs of a Ground Source Heat Pump?

A four-bedroom house is likely to need around 11,000kWh of heat for space heating and 4,000kWh for domestic hot water. 

If we assume a SCoP (Seasonal Coefficient of Performance) of 4.5, then the property will need (11,000 + 4,000)/4.5 = 3,334kWh of electricity to run it.

Electricity at around 15p/kWh (including VAT, standing charges and so on) gives a running cost of £500 per year

That compares to £890 per year to run a gas boiler and around £750 for an air source heat pump.

a ground source heat pump horizontal array with straight pipes

This horizontal ground array is being installed in straight pipes, rather than looped slinkies.  (Image credit: Kismet Photography)

Load-Shifting Ground Source Heat Pumps 

Load-shifting is a strategy for managing energy use by shifting from peak hours to off-peak hours where the energy source costs less. Ground source heat pumps are a prime candidate for applying load-shifting strategies, as they can run at off-peak times with ease, lowering the cost of the electricity it requires to run. 

This is because:

  • They're quiet, meaning they're unlikely to cause disturbance if active during the night, for example
  • Ground temperatures stay pretty consistent throughout the day 

Ground source heat pumps with smart controls are able to create a heating schedule based on predicted electricity tariffs, meaning taking advantage of the lowest cost, lowest carbon electricity doesn't require manual operation of the heat pump unit. 

Advances in technology, which include heat storage batteries, also mean that you can save any heat generated during these off-peak times for use during peak hours of the day. 

How Much Money Could I Save by Installing a Ground Source Heat Pump?

The running cost of a ground source heat pump that will be around 3.4p/kWh, while a gas boiler is around 6p/kWh (as it will vary with different suppliers and assuming 90% efficiency) and an air source heat pump will be around 5p/kWh

How much you actually save will obviously vary with how much heat the house needs. 

a slinkie trench for a ground source heat pump array

Coiled pipes, known as slinkies, are one option for your horizontal array.  (Image credit: Kensa Heat Pumps)

What Grants are Ground Source Heat Pumps Eligible For?

Ground source heat pumps qualify for the government’s Renewable Heat Incentive  which will close to new applicants on 31 March 2022. 

Under this scheme, those with renewable heat-generating technologies are paid back for the heat they produce.

The RHI scheme will pay 21.16p/kW for each kWh of heat produced by the heat pump (less the electricity used to produce it), compared to 10.85p for air source and, of course, nothing for a gas or oil boiler. 

The Renewable Heat Incentive will be replaced by the Clean Heat Grant, which focuses specifically on heat pumps

How Do Ground Source Heat Pumps Work?

A mix of water and anti-freeze is pumped around the ground array to absorb the low-grade heat provided by solar energy and stored in the ground. 

The heat pump uses compression and expansion technology (as your fridge does), to extract and amplify that heat, increasing the temperature and making it useful in the house.

The amount of heat that can be collected will be dictated by the soil conditions — for example, clay holds more heat than sand — and the amount of unshaded land available. 

Are Ground Source Heat Pumps Noisy?

The loudest ones would be around 42 decibels, equivalent to a chest freezer.

As a rule of thumb an area of 50m2 is needed for each 1kW of heat output; an 8kW heat pump will need 400m2, assuming damp clay soil (a tennis court is 260m2). 

A good installer will calculate the exact amount of heat needed for the house to determine the size of the heat pump needed. 

They will also will investigate the ground conditions to determine the type of soil, if there is sufficient space for a horizontal array, or if boreholes will be necessary.  

a borehole ground source heat pump during installation

A borehole ground source heat pump is perfect for compact sites, but requires specialist digging equipment to install.  (Image credit: Kensa Heat Pumps)

What Size Ground Source Heat Pump Do I Need?

Big enough to heat the house, and no more. A modern 4-bedroom house is likely to need an 8kW heat source but could often have a 20kW boiler installed. 

There are commercial and technical reasons for this oversizing, but a heat pump is not the same. 

In that situation the heat pump would need to be 8kW and no more, to achieve maximum efficiency.

How are Ground Source Heat Pump Pipes Installed?

Can I Install a Ground Source Heat Pump Myself?

No, it's not advisable, for two reasons. Installing both the heat pump and the ground array are technically complex task, best left to properly trained and experienced people. And, to qualify for RHI the system must be installed by an MCS-accredited supplier. 

For a horizontal ground array:

The pipes in the ground (the ground array) can be either straight or coiled pipes (slinkies). There is no technical or efficiency difference between the two and the choice will be dictated by practical on-site issues. 

In either case the pipes will ideally be 1.2m below ground level. As important as depth is the spacing between the pipes. 

As the horizontal ground array is collecting heat introduced to the ground by the sun, pipes need to be spaced sufficiently far apart to ensure they do not take more heat than they should and chill the ground. 

That is 3m for straight pipes and 5m for slinkies (assuming clay soil, wider for other soil types). In addition, pipes need to be at least 5m from the boundary with any adjoining property. 

a borehole being drilled for a vertical array ground source heat pump

Specialist drilling equipment is required to create boreholes for a vertical array.  (Image credit: Kensa)

For a vertical, or borehole system: 

These will be anything from 70m to 120m deep, depending on the ground conditions and the size of the heat pump. As will the number of boreholes needed. 

As the temperature of the ground rises with depth it is often advantageous to have fewer, deeper boreholes, but that is not always possible. 

Do I Need Planning Permission to Install a Ground Source Heat Pump?

There is nothing externally visible and it is usually considered Permitted Development so you shouldn't require planning permission

If the property is in a conservation area or overlooked by a listed property, it is wise to consult the appropriate authorities.  

a plant room containing a ground source heat pump unit

Best practice is to house a ground source heat pump unit in a dedicated plant room or utility room. Access should be easy for any maintenance requirements.  (Image credit: Viessmann)

How Efficient are Ground Source Heat Pumps?

Efficiency is stated as the SCoP and a typical figure for a ground source heat pump would be something over 4.0.

This means that for each 1kW of electricity used to drive the heat pump, it will produce 4.0kW of heat. It is primarily this efficiency that make heat pumps a good idea. 

The key issues to low running costs and high efficiency are:

  • A well-insulated house will need a smaller heat pump, a smaller ground array (or fewer boreholes) and less electricity, reducing capital and running costs. System efficiency therefore starts with minimising the amount of heat required in the house
  • A well-designed system — specifying a heat pump is more complex than specifying a gas boiler. It is common to over-specify the size of your gas boiler ‘just in case’, this is not a good idea when it comes to heat pumps. An over-sized system is more expensive and operates less efficiently than the right sized one. 

installation of a ground source heat pump

Installing a slinky-piped horizontal ground array for a ground source heat pump system. (Image credit: Kensa Heat Pumps)

It has to be accepted that installing a ground source heat pump is not a DIY project.

It is a technically complex task that requires trained and experienced people. In addition, to qualify for RHI the installer and the equipment need to be MCS certified. 

The key to all this is finding a trustworthy installer who will design and size your system and a good starting point is to choose a member of the GSHP Association. The indicators of a good installer would be someone who checks the soil conditions and offers quality equipment.  

But it usually comes down to due diligence: checking the installer’s history and the references they supply. 

Can You Use a Ground Source Heat Pump with Radiators?

The short answer is yes. 

Radiators are designed to work with the high flow temperature available from a gas or oil boiler – usually something over 70°C. Heat pumps are most efficient with a flow temperature below 45°C. 

Heating systems usually cycle – switch on and off – as the room cools and more heat is needed. The generally accepted solution is to increase the size of the radiator (probably double) so that the room is heated in a reasonable period of time. 

There is a school of thought, now gaining some traction, that suggests that even at 45°C the radiator will eventually warm the room, and that if the heating system is run 24/7 the room will not cool and bigger radiators are not needed. 

And as the system is still putting in the same amount of heat there is no effect on running costs. 

(MORE: How to Replace a Radiator)

What Maintenance Does a Ground Source Heat Pump Need?

Heat pumps are not maintenance free, but as they are sealed systems the maintenance burden is fairly low. 

Like any heating system they need an annual check of the main components – pumps and motors – and to ensure no refrigerant has leaked. 

Cost is likely to be £100 to £150 per year. 

Is a Ground Source Heat Pump Right for My Home?

If you are considering a ground source heat pump, bear in mind:

  • A heat pump works best in a well-insulated home. A heat pump would still work in a home with a high heat demand will but it would be more expensive. The point is that the difference in price between a, say, 15kW and 30kW gas boiler might be £1,000, including installation. The difference between a 10kW heat pump and a 20kW might be £10,000, plus the extra installation costs. Investing in insulation has a significant impact on the cost of the GSHP.
  • A horizontal array needs enough garden space. That said, a borehole installation is an option for those short on space. You will also need to consider access to the site for excavation machinery
  • It will need to be paired with a suitable heating system and is most efficiently paired with underfloor heating as this requires much lower water temperatures than radiators. The alternative is to upgrade existing radiators to cope with the lower flow temperature

They offer a high RHI return, low running and maintenance costs and systems are more reliable than a gas boiler and longer-lasting. The question is perhaps not “is my house suitable for a heat pump” but rather “how do I make my house suitable”?  

Can Ground Source Heat Pumps Provide Cooling?

Some ground source heat pumps can be optimised to also provide passive cooling to your property with the addition of some extra modules. Just as a ground source heat pump uses the natural warmth of the earth to heat a home, a ground source heat pump being used for passive cooling uses the temperature difference between the earth and the inside room temperature. 

This is achieved by using systems such as fan coils or passive beams, passing the fluid of the cooling system through a plate heat exchanger with the ground array fluid passing through the other side.

Some air source heat pumps also include a reverse cycle modes, which can be used to provide active cooling through air conditioning. 

Are Ground Source Heat Pumps Worth it?

Depends on your point of view. Many find the high capital cost unpalatable. Others find the low running costs, high RHI payments, longevity and low CO2 emissions highly attractive. 

It is reasonable to suggest that properly installed in the right situation there is no better alternative.  

Tim Pullen

Tim is an expert in sustainable building methods and energy efficiency in residential homes.