Ground source heat pumps may seem complex, but they are quite simple bits of kit. Here we explain what you need to know before investing in one for your home.

Ground source heat pumps absorb solar energy — more specifically, the energy from the sun warming the ground. They consist of a series of pipes buried underground which extract this energy. This, in turn, is converted into heat for use in the home.

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

  • The ground array – which can be either vertical or horizontal – which is buried underground.

A horizontal ground array being installed (by Kensa Heat Pumps).

  • The noticeable bit in the house is the heat pump itself. Some of these include a hot water cylinder and can be as big as 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.

The heat pump unit may need to be installed in a dedicated plant room (again, this unit is by Kensa Heat Pumps).

How Do Ground Source Heat Pumps Work?

A mix of water and anti-freeze is pumped around the ground array; this absorbs the solar energy stored in the ground. Heat is extracted using compression and expansion technology and the energy is made available to heat the house.

The amount of heat available to be collected will be impacted by capacity (the amount of piping and the length of the trenches needed) and the soil conditions — for example, clay holds more heat than sand.

A good installer will investigate the ground conditions before designing and sizing a ground source heat pump system.

How are Ground Source Heat Pumps Installed?

The choice between a horizontal and vertical array will depend on the space available and your ground conditions.

  • A horizontal array consists of a pipe laid in a serpentine closed loop in a trench, usually 1.2m deep. You will need a fairly large garden — allow 500m² for a 10kW heat pump in clay soil, and twice that for sandy soil. Pipes can be a straight pipe or in coils, called a ‘slinky’ pipe.

A horizontal ground array shown (by Worcester Bosch).

  • A vertical array, often called a borehole system, will have a number of boreholes drilled into the ground and connected across their tops, in a closed loop. 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 70m to 100m deep (or two slightly deeper boreholes). Their big advantage is that they do not need a large area of land and if the price is right then a vertical array is a good option.

A vertical or borehole array shown (by Worcester Bosch).

  • A final but rather uncommon option is that heat can also be extracted from lakes or ponds, if the body of water is big enough. But this is a more specialist area and requires careful calculation, usually carried out by the heat pump manufacturer.

How Much Does a Ground Source Heat Pump Cost?

The Energy Saving Trust estimates that the cost of installing a typical ground source heat pump system in a three to four bedroom house will be over £10,000.

The generally accepted ‘budget’ figure is £1,200 per kW capacity. A 200m², four-bedroom house, built to Building Regulations’ standard is likely to need an 8kW heat pump. The machine itself is likely to cost around £6,000-£7,000, the balance being the installation cost which can vary significantly with the ground conditions.

It’s worth noting that there is a ‘postcode’ lottery element to vertical arrays, as the cost of borehole digging varies enormously around the country. The price can vary from reasonable to very expensive.

Installing the slinky pipes for a ground source heat pump

Installing a slinky-piped horizontal ground array for a ground source heat pump system.

How Efficient are Ground Source Heat Pumps?

Heat pumps run on electricity and that will always be an expensive form of energy.

Efficiency is stated as the SCoP (Seasonal Coefficient of Performance). A typical figure for a ground source heat pump might be 4.0. This generally means that for each 1kW of electricity used to drive the heat pump, it will produce 4.0kW of heat.

So, the key to low running costs and efficiency is:

  • 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 in this instance and the amount of electricity needed.
  • A well-designed system — specifying a heat pump is arguably more complex than specifying a gas boiler. Whereas you may over-specify the size of your gas boiler ‘just in case’, this is not a good idea when it comes to heat pumps. Over-sized systems are more expensive and, perhaps counter-intuitively, one that is too big will be less efficient.

The key to all this is finding a trustworthy installer who will design and size your system. There are indicators of a good installer – someone who checks the soil conditions and offers quality equipment, for instance – but it usually comes down to due diligence: checking the installer’s history and the references they supply.

Another good starting point is a member of the GSHP Association.

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 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.

The Renewable Heat Incentive

Ground source heat pumps qualify for the Renewable Heat Incentive (RHI) — a government scheme under which those with renewable heat-generating technologies are paid back for the heat they produce.

From 1 April 2019, the rate for domestic ground source heat pumps is 20.87p/kWh (which has improved — it was 20.46p/kWh between April 2018 and March 2019), payable for seven years from the date of commissioning.

But remember, RHI payments are for the renewable element of the heat produced, not for the electricity used to produce it.

In the scenario above, we need 11,000 + 4,000kWh of heat, less the 3,334kWh of electricity used, so RHI applies to:

  • 11,666kWh at 20.87p/kWh = £2,435 per year for seven years

In other words, you could be receiving nearly £2,000 of ‘profit’ once you have paid your electricity running costs.

Is a Ground Source Heat Pump Right for Your Home?

If you are considering a ground source heat pump, you need to ensure the following:

  • There is enough garden space for a horizontal array. 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.
  • A heat pump works best in a well-insulated home. A home with a high heat demand will need a larger system which will be expensive to buy, expensive to install and very expensive to run.
  • It will need to be paired with a suitable heating system and is most efficiently paired with underfloor heating. This is because underfloor heating requires much lower water temperatures than radiators.

The Benefits

Ground source heat pumps are expensive to install but they offer a high RHI return and the lowest running cost of any renewable energy. What’s more, systems are generally reliable and long-lasting.

The key is to find a good, qualified, experienced installer. That will ensure the system is designed to properly meet the ground conditions, and the demands of the house, and will run happily for 20 to 30 years.

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