Ground source heat pumps have been making headlines this year with the launch of the Boiler Upgrade Scheme in April. And the role that low-carbon heating solutions such as heat pumps are likely to play in the future of our home’s heating is becoming increasingly evident, particularly as energy prices rise and climate change heightens.
The cost of heat pump technology and how much the government will have to subsidise for them to become a realistic replacement for gas boilers have been common concerns. While the Boiler Upgrade Scheme goes some way to aid, ground source heat pumps are still a considerable investment, and are not an option for every home.
However, for the homes that do suit the use of a ground source heat pump, those armed with the right information and expectations can expect an efficient, long-lasting low-carbon system, with little maintenance requirements.
This complete guide to ground source heat pumps covers how they work, the costs and what you need to know before investing in one for your home.
What is a ground source heat pump?
Ground source heat pumps may seem complex, but they are quite simple bits of kit. They use a series of buried pipes 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 to 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 units include a hot water cylinder and can be the size of a large filing cabinet. The smallest pumps can be fitted in a kitchen cupboard, however, they're often located in dedicated plant rooms for easy access. Heat pumps are also generally installed close to an external wall to give easy access to the ground array pipes.
Ground source heat pumps require electricity to work. However, they 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, making them an even greener option.
How do ground source heat pumps work?
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A mix of water and anti-freeze is pumped around the ground array pipes 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 ground loop of any ground source heat pump is made up of a number of pipe loops – anything from 2 loops to 12 loops – that are connected to a manifold in a bespoke chamber in the ground. There are then two slightly larger bore pipes that then connect the manifold to the heat pump unit 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.
As a rule of thumb, an area of 50m2 is needed for each 1kW of heat output. So an 8kW heat pump will need 400m2, assuming damp clay soil. (For context, a tennis court is 260m2.)
Before installing a ground source heat pump, a good installer 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.
Find out more about how ground source heat pumps work with our video…
What types of ground source heat pump are available?
There are three main types of ground source heat pump array or installation available:
- Horizontal ground array
- A vertical (or borehole) ground array
- Water source heat pumps
A horizontal ground array will be either straight pipes or coiled pipes (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.
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 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. These will be anything from 70m to 120m deep.
As the temperature of the ground rises with depth it is often advantageous to have fewer, deeper boreholes, but that is not always possible. As an example, 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.
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.
What are the advantages of ground source heat pumps?
There a good number of benefits to investing in this low-carbon heating solution, including:
- Running cost is likely to be around 45% lower than for a gas boiler, while also significantly less expensive than other renewables, including air source heat pumps.
- 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, namely the Boiler Upgrade Scheme, available to offset the cost of ground source heat pumps.
What are the disadvantages of ground source heat pumps?
- The high capital cost of installing a ground source is prohibitive to many.
- There will be significant disturbance to the garden, particularly when installing a horizontal ground array.
- You'll also need to factor in the cost of upgrading your home's insulation, if required, as well as the cost of replacing radiators for low-flow versions or underfloor heating.
How efficient are ground source heat pumps?
Efficiency is stated as the SCoP (seasonal coefficient of performance) 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.
How much does a ground source heat pump cost?
A basic ground source heat pump costs between £2,000 to £15,000 depending on size and brand. This cost is likely to be three to four times more expensive than a gas combi boiler and double the cost of an air source heat pump.
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.
The quality of the heat pump equipment can add a significant amount to the cost of materials, effectively doubling it in some cases.
Top-end heat pumps can have higher quality components and complex on board software for controlling and monitoring all aspects of the heat pump operation and also more costly alloys are used in the construction of the cases and components
The ground loop and installation complimentary systems (such as underfloor heating and radiators), as well as other peripheral materials, can substantially add to the cost of a ground source heat pump system, too.
How much do ground source heat pumps cost to install?
The complete ground source heat pump installation for a 240m2 home will range in cost from around £15,000 to £25,000, including the unit.
There are a number of factors which impact the cost of a ground source heat pump installation, including:
- The complexity of the installation, including the distance from the heat pump unit to the ground loop manifold chamber will add to the installation cost.
- The type of installation — more specifically, whether a horizontal or vertical array is required.
- Bigger properties will need larger heat pumps, even possibly more than one unit, alongside a more complex hydraulic design.
Horizontal ground array ground source heat pump
The cost of a horizontal ground array will depend a lot on the ground conditions. A very wet environment will possibly require pumps to clear the water in a trench while the pipes are laid. The walls of the trench could also be unstable which will require further support and a resultant cost increase.
If there are a lot of rocks in the ground then this will also add cost to the excavation.
The cost of the pipework for a 500m2 ground loop is around £2,000 and the cost of the excavation could be around £300 per day for a digger driver and £300 per week for the digger. Allow around £4,000 – £6,000.
Borehole/vertical ground source heat pump
Borehole collectors are more expensive than horizontal ground loops. When done properly with a geologist report, it will cost around £100 per meter. There will usually be around 10 meters of borehole per kW so a 12kW heat pump will need around a 120 meter borehole or two 60 meter boreholes.
The overall cost, with the pipe installed and properly backfilled with the correct grout (often Bentonite clay), will be around £12,000.
What are the additional costs of retrofitting a ground source heat pump in an existing home?
"Heat pumps operate at low temperatures which are much lower than the high system temperatures associated with oil boilers. Therefore, to help a heat pump work both efficiently and effectively, high levels of insulation ideally need to be in place so that any heat loss from the property is minimised," says Neil Sawers, Commercial Technical Manager at Grant UK.
Many older properties are not airtight and have sub-optimal levels of insulation, resulting in lower efficiency and resultant higher running costs. This means a requirement to spend on upgrading the insulation and airtightness of the property before installation of a ground source heat pump can be considered.
On top of this, you may also require new pipework to the hot water cylinder, and to the central heating distribution which may be located near the existing boiler rather than near the proposed location of the heat pump unit. A new hot water cylinder may be required too.
Even if you have a wet radiator system already, you may need to upgrade the size of the radiators to accommodate the low flow temperatures of a heat pump system to ensure it runs efficiently.
With the potential costs of replacing radiators and fitting additional insulation to improve your home's fabric enough for a ground source heat pump to be a viable option, the overall cost of installation will increase dramatically. This could see overall installation costs grow to £35,000 or more.
Anticipate that there may be a cost in removing the existing boiler, too. Where replacing oil or LPG heating fuels, there will also be a cost to remove the fuel storage tanks, concrete base and fuel pipework.
What are the running costs of a ground source heat pump?
In a property with an annual heat load of 15000kWh, you would expect the electric use to be somewhere between 3600kWh and 4700kWh.
Based on the cost of a unit of electricity at around 28p per kWh and the average efficiency of a ground source heat pump at 320% to 420%, this represents an annual running cost of between £1,008 and £1,316.
Can load-shifting help save money?
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 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.
What ground source heat pump grants are available?
The Boiler Upgrade Scheme is the main grant available for the installation of ground source heat pumps on domestic properties. It's the successor to the Renewable Heat Incentive — which closed on 31 March 2022 and provided those with renewable heat-generating technologies payment for the heat they produced over a period of seven years.
The Boiler Upgrade Scheme launched in April 2022 and offers a fixed upfront payment towards the installation of a heat pump or biomass boiler. Under the Boiler Upgrade Scheme, homeowners will be able to claim up to £6,000 towards the installation of a ground source heat pump. This will be issued in a voucher system, similar to how the Green Homes Grant once operated.
The payment available for ground source heat pumps is £1,000 more than for air source heat pumps, but this reflects an installation cost which are potentially a lot higher.
There are several other heat pump grants around the UK which can be used to recoup the cost of installation of a ground source heat pump. However, these are typically open to households on lower incomes.
How big should a ground source heat pump unit be?
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 a gas boiler, 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.
A good installer will calculate the exact amount of heat needed for the house to determine the size of the heat pump needed.
Do I need planning permission to install a ground source heat pump?
If the property is in a conservation area or overlooked by a listed property, it is wise to consult your local authority first.
What are hybrid ground source heat pumps?
Hybrid heat pumps are, in essence, one system that contains both a heat pump and a boiler. The benefits of these systems are that the right heating system can be used to suit the right temperature, helping to reduce running costs, while also keeping carbon emissions as low as possible.
Due to the low-temperature nature of ground source heat pumps, they're better suited to heating a well-insulated home at a milder temperature, while when the temperature drops very low outside, the requirements of the GSHP to reach the heating levels required makes it less efficient.
In a hybrid heat pump, at this point a boiler would kick in, ensuring that gas (and potentially other fuels such as hydrogen in the future) are used where that's the more efficient option.
Can I install a ground source heat pump myself?
No, it's not advisable — it has to be accepted that installing a ground source heat pump is not a DIY project. Installing both the heat pump and the ground array are technically complex tasks, best left to trained, experienced installers.
What's more, to qualify for Boiler Upgrade Scheme, the system must be installed by an MCS-accredited supplier.
Those well-versed in groundworks, or using a digger and dumper, may wish to dig the ground trenches for a horizontal array, but the installation should be left to the professionals. Digging boreholes for vertical arrays requires specialist equipment and should be left to the professionals too.
How do I find a good installer?
The key to installing a ground source heat pump 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 and/or use a MCS registered installer.
The indicators of a good installer would be someone who checks the soil conditions and offers quality equipment. But it also 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. However, these need to be low flow radiators, sized and designed to work with heat pumps. They are typically larger than standard radiators.
This is because standard 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.
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. The cost is likely to be around £100 to £200 per year.
Are ground source heat pumps noisy?
All heating products make some noise, but heat pumps are usually quieter than fossil fuel boilers. The loudest ground source heat pumps may reach 42 decibels — equivalent to a chest freeze.
To put this in context a modern boiler should be around 40-60 decibels.
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.
Is a ground source heat pump worth it?
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. 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 standard radiators. The alternative is to upgrade existing radiators to cope with the lower flow temperature.
Whether a ground source heat pump is worth it depends on your point of view. Many find the high capital cost unpalatable. Others find the potentially lower running costs, upfront Boiler Upgrade Scheme payment, low CO2 emissions and highly attractive.
While the cost of a ground source heat pump does dwarf a gas boiler, it’s also worth factoring in the longevity of these systems versus that of a standard combi boiler.
It’s realistic to suggest that in many instances, a ground source heat pump won’t save you much money compared to a gas boiler. That’s not necessarily the point of transitioning from a gas boiler to a heat pump at present, but it’s something that will hopefully change in the near future, as the government looks at moving levies away from electricity to gas, bringing down the cost of electricity to operate GSHPs.
It is reasonable to suggest that properly installed in the right situation there is no better alternative when it comes to sustainable heating for the home.
They offer 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”?
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Tim is an expert in sustainable building methods and energy efficiency in residential homes and writes on the subject for magazines and national newspapers. He is the author of The Sustainable Building Bible, Simply Sustainable Homes and Anaerobic Digestion - Making Biogas - Making Energy: The Earthscan Expert Guide.
His interest in renewable energy and sustainability was first inspired by visits to the Royal Festival Hall heat pump and the Edmonton heat-from-waste projects. In 1979
this initial burst of enthusiasm lead to him trying (and failing) to build a biogas digester to convert pig manure into fuel, at a Kent oast-house, his first conversion project.
Moving in 2002 to a small-holding in South Wales, providing as it did access to a wider range of natural resources, fanned his enthusiasm for sustainability. He went on to install renewable technology at the property, including biomass boiler and wind turbine.
He formally ran energy efficiency consultancy WeatherWorks and was a speaker and expert at the Homebuilding & Renovating Shows across the country.