Ground source heat pumps may sound complex, but did you know you already have the same technology in your home in your fridge or freezer?
Energy efficient, reliable and renewable, ground source heat pumps have a lot to offer in the move away from our homes reliance on fossil fuel power. Heat pumps are also subject to an increasing number of government schemes aiming to increase adoption of these eco heating systems, including the Renewable Heat Incentive (RHI) and Green Homes Grant, as well as the proposed Clean Heat Grant.
However, it’s not a one-size fits all solution to supplying homes with green heating. From how ground source heat pumps operate to the cost implications and suitability of your home, we take an in-depth look at this renewable technology.
(MORE: Get a quote for your heat pump)
What is a Ground Source Heat Pump?
Ground source heat pumps (GSHP) are a way of heating your home with energy from the sun, but in a very different way to solar panels. GSHPs 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.
Ground source heat pumps use electrical energy in a highly-efficient way to produce heat.
The effect of this is to significantly reduce heating running costs and also to reduce CO2 emissions.
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.
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How Much Does a Ground Source Heat Pump Cost to Run?
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.
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.
What Grants are Ground Source Heat Pumps Eligible For?
Ground source heat pumps qualify for the government’s Renewable Heat Incentive which is currently slated to 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.
They are also eligible as a primary measure under the Green Homes Grant. This sees the government offer vouchers up to the value of £5,000 (or £10,000 for certain low income households) towards the cost of installing a range of green home measures, including ground source heat pumps.
While RHI and the Green Homes Grant can both be used on the same ground source heat pump installation, the value of any vouchers from the GHG will be subtracted from subsequent RHI payments.
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.
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.
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 Pumps Installed?
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.
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.
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 Deep do the Pipes in the Ground Need to be Installed?
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.
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?
If the property is in a conservation area or overlooked by a listed property, it is wise to consult the appropriate authorities.
What are the Pros and Cons 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
- 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
- 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
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.
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”?
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.
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