Ground Source Heat Pumps (GSHP) are becoming an increasingly popular technology. Society is transitioning away from fossil fuel heating systems (gas and oil-fired boilers), causing an acceleration in demand for low and zero carbon solutions.

With a GSHP system, it is important to accurately calculate the number and depth of boreholes needed. If too few boreholes are drilled, or if the boreholes are not deep enough, the system will be inefficient and won’t provide enough heat to the property.

Why is a Thermal Response Test needed?

The specification for the borehole array partly depends on the property itself (size of the property, insulation, window glazing etc). It also depends on the thermal properties of the geology. To understand these thermal properties, a Thermal Response Test can be carried out.

A Thermal Response Test (TRT) is often carried out when the geology beneath the site is not well-known, or where the heat demand for the property is expected to be especially high (large houses, schools, hospitals, etc).

How is a Thermal Response Test carried out?

A borehole will be drilled to a pre-determined depth, usually between 100 and 200 metres below ground level. Once drilled, a geothermal collector (usually 40mm polyethylene pipe with a u-bend) is installed to the depth of the borehole. The collector loop is pressure-tested after installation and the borehole is backfilled with thermal grout.

Once the drilling and installation is complete, it will then be time to run a Thermal Response Test on the geothermal borehole. This involves circulating a special fluid into the geothermal collector loop to gather data on the thermal performance of the borehole. This includes its thermal conductivity and resistance.

This data is then analysed, with recommendations for the full GSHP system presented in a summary report. These recommendations will help the designer to more accurately calculate the number and depths of boreholes required for the full GSHP system.

What should be the duration of a Thermal Response Test?


A Thermal Response Test typically runs for 48 hours, however the duration may be longer depending on the specific needs of a project.

When should a Thermal Response Test be carried out?


A Thermal Response Test is usually required early in the process, so the designer can calculate the requirements for the Ground Source Heat Pump system and boreholes.

The most cost-effective and convenient time to conduct a TRT is usually when a geotechnical ground or mining investigation is taking place on site. This is because the same drilling rig needed for these investigations can often be used to drill the geothermal borehole too. This reduces mobilisation costs, resulting in an overall saving for the client.

If a TRT is not carried out during the ground investigation, it can still be carried out later as a standalone service. In our experience it makes sense, however, to carry out the TRT earlier in the process.

Can Hydracrat carry out Thermal Response Tests?


Hydracrat can arrange for a full Thermal Response Test to be carried out. This includes the drilling, installation of the geothermal materials, on-site Thermal Response Test and provision of the TRT report.

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Ground Source Heat Pump boreholes are the most common method for extracting heat from the ground to warm properties.

As you’ll probably have noticed, heat pumps have been making an increasing number of headlines in the run-up to the COP26 UN summit in Glasgow. Our team decided, therefore, that it was time to help answer some of the key borehole-related questions!

Can’t see your question on the list, or want some advice about the potential for a GSHP on your property? Contact us.


There are two types of Ground Source Heat Pump boreholes, known as “closed-loop” and “open-loop.”

Closed-Loop boreholes are designed to harness heat from the ground by circulating thermal transfer fluid within plastic pipes which are installed in boreholes.

The plastic pipes, known as ‘collector loops’, are fitted with a u-bend.

The thermal transfer fluid will be relatively cold as it enters the borehole but is warmed by the ground temperature as it circulates to the u-bend at depth and back up towards the surface.

When the fluid exits the borehole it is circulated towards the heat pump, which effectively extracts the heat from the fluid. The fluid, now cooler, is circulated back towards the borehole again where it repeats the cycle.

Hence the term “closed-loop”!

Our example demonstrates the typical design of a closed-loop borehole.

Open-Loop boreholes are designed to harness heat through the abstraction of groundwater from aquifers or underground mine workings. The differences between closed-loop and open-loop boreholes are explored in a dedicated section on this page.

Closed-loop boreholes are the more common solution but Open-loop systems can be especially viable on developments with an especially high demand for heat. This can include district heating schemes or industrial warehouses and factories.

Borehole for Ground Source Heat PumpBoreholes for a Ground Source Heat Pump are drilled and installed by a specialist drilling contractor, such as Hydracrat.

The drilling contractor is typically commissioned by an engineer, installer or designer to carry out these works on a sub-contract basis.

The drilling contractor will set up the drilling rig at each of the borehole locations and drill vertically into the ground until the target depth is reached. Depending on the heat requirements of the property, each borehole will likely be between 100 and 150 metres deep.

The drilling contractor will typically supply the materials to be installed within the boreholes, including the collector loops.

These loops shall be pressure-tested before installation and subsequently installed within the borehole to the final depth. The loops are then pressure-tested once more, to ensure that the pipes have not been damaged during the installation.

The borehole will then be backfilled with grout to protect the loops and enhance thermal potential.


Closed-Loop Boreholes


  • Boreholes usually drilled to depths of between 100 and 150 metres below ground level.
  • Boreholes typically drilled in 120mm (4 ¾ inches) diameter.
  • Polyethylene collector pipe installed to depth of borehole. Pipe usually either 32mm or 40mm in diameter with u-bend at the final depth of the borehole.
  • Boreholes backfilled with grout to surface (backfill specification determined by designer and/or installer).


Open-Loop Boreholes


  • Boreholes drilled to target productive aquifer, usually to depths of between 60m and 200 metres below ground level. Boreholes sometimes drilled to greater depths to target abandoned coal mine workings.
  • Boreholes drilled in larger diameters to 300mm (12 inches) or greater. This can help to maximise the water abstraction potential.
  • Liner installed within the borehole to final depth, to restrict sediment infiltration.
  • Submersible water pump, electric cabling, riser pipe and other equipment installed within borehole. This equipment will be specified based on a target rate of groundwater abstraction.
  • Separate borehole(s) will be required to re-inject the (cooler) water back into the same aquifer.

An average sized house in Scotland will usually need 2-4 boreholes each drilled to depths of between 100m and 150m below ground level.

The number of boreholes and the depths of these boreholes are specific to each property and depend on many different factors. The most critical factors are:

Size of the Heat Pump

The load of the heat pump (expressed in kilowatts or kW) is clearly a critical factor.

This will be determined based on the estimated “heat loss” of the property based on factors such as the room sizes, volume/quality of insulation and the type of windows.

Generally the larger the property, the higher the heat pump load needs to be in order to meet the heat demands of that property, but this is not always the case.

Thermal Conductivity of the Geology

The Scottish geology is incredibly complex and can vary significantly between the different regions.

Each type of rock offers a different level of thermal conductivity. In layman’s terms, that’s the capacity of the rock to store and transfer heat to the collector loop within the borehole. Geology which offers better thermal potential may not require boreholes to be quite as deep, compared to boreholes in areas where the geology is less favourable.

The thermal properties of the ground in the context of designing boreholes for a GSHP are often calculated and expressed in Watts per metre (W/m) of borehole length.

An average sized house will usually require a 10-15kW Heat Pump. This in turn will typically require 2-4 boreholes each drilled to a depth of around 100-150 metres.

Your drilling contractor and installer will be able to carry out an exercise to calculate the number and depth of the boreholes, whilst providing you with the budget price for the installation works.

It’s important not to take shortcuts when it comes to defining the requirements of a Heat Pump and the borehole array. Proper consideration will help avoid mistakes and costly rectification works later.

A closed-loop borehole may offer anything from 3-7kW but this depends on many factors including the depth of the borehole, materials installed within the borehole and the thermal conductivity of the surrounding rock.

It’s important to determine the required load for the GSHP first, as this will then help dictate what the specification of the borehole(s) should be.

A qualified designer/installer (alongside the drilling contractor) will be able to carry out the heat loss calculations to establish the required load of the GSHP. In turn, this will inform the project team how much drilling will be required.

The cost to drill and install the boreholes varies from project-to-project.

The cost of these works are determined based on a number of factors, including:

  • Cost of transporting drilling rig, equipment and crew to site.
  • Target depth of the borehole.
  • Depth of superficial deposits which are overlying the bedrock (essentially the depth of the topsoil, sands, gravels, clays or made ground which sits above the natural bedrock).
  • Type of bedrock (some rock may be harder and thus slower to drill). The geology in Scotland varies significantly and as such, so do the drilling costs.
  • Type of installations and materials required within the borehole.
  • Other factors such as the potential for drilling through disused coal mine workings and the expected groundwater conditions will also have an impact.

Contact us if you’d like to know more about the likely costs of a GSHP with boreholes at your property.

This is an important factor when considering whether a Ground Source Heat Pump is suitable for your property. A few of the key considerations are:

Space for Drilling Rig / Crew

It’s important to make sure that there is sufficient space for the drilling rig to access the drilling location. This basically means making sure that any gates, paths or driveways are wide enough for the drilling rig to be moved safely into position.

The drilling contractor will also need to confirm there is sufficient space to store equipment on site and whether there is enough safe working space for the crew.

Borehole Spacing

Your property will require a total length of collector pipe. To achieve the total collector pipe length, the pipes are usually split into collector loops, which are installed within the vertical boreholes.

Where more than one borehole is required, each borehole will need to be spaced sufficiently apart from each other (usually 6-8 metres) so that heat extraction from the collector loop in each borehole won’t negatively affect the performance of the others.

Your installer / designer will need to make sure that adequate borehole spacing can be achieved within your property.


Boreholes will also need to be located in areas which are free of any underground services such as electric cables, gas or water pipes.

Mine workings

Properties in some areas of Scotland may be sitting above abandoned underground coal/shale mine workings. These mine workings may exist at sufficiently shallow depth that any proposed boreholes could intercept them.

Mine workings can have an impact on the design and installation costs of installing closed-loop boreholes for GSHPs, so it’s important to understand the risks in advance.

Your installer, designer or drilling contractor will be able to help indicate if boreholes at your property are likely to encounter potential mine workings.

Need some advice on the prospect of drilling boreholes at your property? Contact us

Hydracrat recommends that boreholes are drilled no closer than 10 metres from existing buildings, where space permits.

If less space is available, a preliminary site visit should be carried out to assess feasibility of drilling.

We also recommend boreholes are spaced at least 3 metres from any boundary with a neighbouring property.

The installer will generally stipulate boreholes must be spaced at least 6-8 metres apart from each other.

This is to prevent possible thermal breakthrough which could impact performance of the GSHP.

For larger systems requiring more boreholes, it may be necessary to have greater spacing.

A properly installed borehole should last for several decades and can last significantly longer, potentially up to 100 years.

It is important to ensure that the drilling contractor employed to carry out the works is reliable and experienced in the field.

Hydracrat are proud members of the Ground Source Heat Pump Association, who have developed a series of standards for the installation of vertical boreholes.

Yes, Ground Source Heat Pumps are becoming increasing popular in Scotland.

GSHPs are an extremely efficient technology and offer the potential for some of the greatest reductions in carbon emissions compared to gas boilers.

All kinds of properties are now adopting GSHPs, from private residences and social housing developments to large commercial buildings, warehouses and factories.

District Heating Systems using GSHPs which have the potential to heat hundreds of homes are also being assessed in many regions of the country.

Installing a Ground Source Heat Pump can bring significant benefits to the property owner.

Consistent Temperatures

In very cold weather, when heating is most needed, a GSHP using a borehole has access to warmer temperatures from the rock in the ground than an Air Source Heat Pump has from ambient air.

The temperature of the bedrock tends to be approximately 10-12 degrees and varies very little, regardless of the time of day or year.

By comparison an Air Source Heat Pump is less efficient, particularly during the Scottish winter when the outdoor temperature s lower and the air is damper.

Heat (and Cool) Storage

GSHP systems, uniquely amongst renewable energy technologies, offer the opportunity to recycle heat energy. Heat energy can be captured when it is freely available in the summer, stored in the ground over the autumn, and released to heat buildings in winter. This singular merit is attributable to use of the ground for Seasonal Thermal Energy Storage, which is an integral part of ground source energy.

By contrast, it is much more difficult and expensive to store heat extracted from the air.

Heat at night for cheaper electricity

The Thermal Energy Storage capacity of the rock surrounding the borehole allows GSHPs to be used efficiently at all hours of day and night – this provides the opportunity to use GSHPs at night when electricity is much cheaper using an economy 10 tariff or similar.

This is not possible with an ASHP, as the temperature is at its lowest during the night.

Less Maintenance

All of the equipment for a Ground Source Heat Pump is either housed inside the property or in the case of the boreholes, underground. The system therefore isn’t exposed to outside elements such as wind, rain, freezing temperatures and debris such as branches or leaves.

On the other hand, ASHPs require regular checks to the air inlet and evaporator, to ensure they are free of leaves or other debris. Plants or weeds growing within the proximity of the inlet can also compromise the unit, if left unchecked.

Reliable Supply of Heat

GSHPs do not suffer the problems of “intermittency” that affect renewable energy from wind turbines, photovoltaic cells or solar thermal panels.

Indeed the Thermal Energy Storage capacity of the rock surrounding the borehole can be used to compensate for the intermittent supply of energy from other renewable sources.

A Long-Term Investment

Although ground source energy generally requires a higher level of investment upfront, the reductions in emissions of greenhouse gases are sizeable and the system is likely to last much longer than other low-carbon systems.

The boreholes, for example, can be expected to last for over 50 years. The GSHPs themselves are very reliable pieces of equipment with a long life.

GSHPs are also generally more efficient than their Air Source equivalent, so are likely to bring savings on running costs in the long-term.

Energy Security

At present, a significant number of Scottish homeowners live in houses which are heated using some form of fossil fuel. This is commonly with a gas boiler.

A hot topic in recent times has been the volatile cost of gas, much of which is imported from overseas. It is therefore a national challenge to try and control the cost of gas to ensure consumers receive a fair price.

Ground Source Heat Pumps, on the other hand, use electricity. As of 2021, 97% of Scotland’s electricity is generated from renewable sources (wind, hydro, solar) right here in Scotland. Thus in theory, consumers are likely to experience less volatility in the cost of running a GSHP compared to a gas boiler.

Need advice? Our team at Hydracrat will be delighted to help.