Ground source heat pumps (GSHPs) may seem complex, and for some they appear to be a step into the unknown, but they are quite simple bits of kit. Here’s everything you’ll need to know if you’re thinking of installing one in your home.

What is a ground source heat pump?

Heat pumps simply move heat from one place to another — and everyone who owns a fridge or freezer already owns one!

A ground source heat pump extracts latent heat from below ground via a loop of pipe that is buried in your garden. The heat from the ground (at 2m deep, there is a relatively stable 8-12oC temperature) is absorbed by a mixture of water and anti-freeze that is circulated through the loop, passed through a heat exchanger into the heat pump, which then feeds the system.

How are ground source heat pumps installed?

You will either have a horizontal ground array installed, or boreholes can be dug for a vertical installation. The choice will depend on the space available and your ground conditions.

Horizontal arrays can come with straight or slinky pipes. People who advocate the use of straight pipes don’t tend to like slinkies, and vice versa. The reality is that both work as well as each other and whatever your chosen supplier is happiest with will be fine.

The key is ensuring there is enough ground to allow pipes to be properly spaced. The calculation for clay soil is 50m2/kW output, so a 14kW heat pump needs 700m2 of unshaded land. Bear in mind that the array needs to be kept at least 5m from any boundary, and that straight pipes need to be 3m apart and slinkies 5m apart.

How much does a ground source heat pump cost?

Cost for a horizontal ground installation will vary with the size of the array, but for a typical domestic property it will be around £3,000-£5,000.

The alternative to a horizontal ground array is boreholes. The cost will be in the region of £3,000-£5,000 per hole. As with horizontal arrays, the amount of heat that can be extracted will vary with the geology — loose stone will have about 20W/m and granite 50W/m.

On the upside, boreholes do not affect the whole garden as a horizontal array will, and there is an efficiency advantage: boreholes drilled 100m deep will deliver up to 5˚C more heat to the pump than a horizontal array. This gives a potential efficiency improvement of 20%.

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 much does a ground source heat pump cost to run?

A house with a heat load of 20,000kWh will need to buy 5,000kWh of electricity to run the heat pump (assuming a Seasonal Coefficient of Performance [SCOP] of 4.0, which indicates that the heat pump will produce 4kW of heat for each 1kW of electricity it uses).

  • At a tariff comparison rate (TCR), which includes VAT and standing charges, of 17p/kWh, that will cost £850.

The current average price of mains gas is about 5.4p/kWh (including VAT), so heating the same house on gas will cost £1,080. The heat pump saves just £230 per year — but don’t forget the Renewable Heat Incentive payment, which is currently 19.64p/kWh.

To work out your tariff payment, it’s your house’s heat load multiplied by 1-1/SCOP then multiplied by the tariff rate. For example, a house with a heat load of 20,000kWh will be paid £2,946 per year — 20,000 x (1-1/4) x 19.64.

What ground conditions do I need?

Heat pumps used to be called (and still are, in some cases) geothermal energy, though this is a misnomer. A ground array (either installed horizontally or vertically via boreholes) collects heat that is introduced to the ground by the sun. It is therefore finite and quantifiable. The amount of heat available to be collected will vary with the type of soil — for example, clay holds more heat than sand.

A good installer will check the ground conditions before sizing and pricing the installation. People who do not check may not be considered a ‘good’ installer.

How boreholes can power your ground source heat pump

How can I make a heat pump more efficient?

Heat pumps are all about efficiency — efficient use of energy and operating an efficient heating system. Heat pumps run on electricity and that will always be an expensive form of energy. System efficiency therefore starts with minimising the amount of heat required and the amount of electricity needed.

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.

You can also buy an inverter, or modulating, heat pump. These pumps vary the speed at which the compressor operates to vary the heat output. A house with a heat load of 20,000kWh would have a peak load of 14kW, so we need a 14kW heat pump.

That peak heat load is calculated to deal with an outside air temperature of (usually) -2°C. Obviously, it is not always -2°C outside so we do not always need 14kW. An inverter heat pump varies the heat output to suit prevailing conditions, saving electricity and improving efficiency.

Also, don’t over specify your heat pump. Over-sized systems are more expensive and, perhaps counter-intuitively, one that is too big will be less efficient.

The key is to find a trustworthy installer. There are indicators – 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.

What heating system is the best pairing for a ground source heat pump?

Underfloor heating is the perfect choice of emitter for a heating system powered by ground source heat pump. This is because underfloor heating requires much lower water temperatures than radiators and a boiler could be said to overheat the water needed for UFH.

Is a ground source heat pump right for my home?

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

  • Your home must have enough garden space for a borehole or horizontal array. You will also need to consider access to the site for excavation machinery.
  • There is no point installing a heat pump of any sort in a home that is not well-insulated. 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 as mentioned above, is most efficient with underfloor heating.
Illustration: Sarah Overs

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