Commercial ground source heat pump: borehole field design, thermal response test and running cost

A commercial ground source heat pump is the premium option for heating a large building: higher efficiency, no external fan noise and a heat source that stays steady through winter. The price of that performance is capital cost, a longer programme and ground works that need proper investigation. This guide explains how the system works, how the ground side is designed, what permits can be involved and when it makes sense. Delivery of these systems is summarised on the commercial ground source heat pump installation page.

How the system works

The core of the system is the same refrigeration cycle as an air source heat pump, but the heat source is different. A loop of buried pipe carries a water and antifreeze mixture. The fluid warms as it passes through the ground, returns to the heat pump and gives up that heat to the refrigerant. The heat pump lifts it to the temperature the building’s heating circuit needs and delivers it through the same buffer vessels, headers and emitters used by any heat pump.

Below a depth of several metres the UK ground stays at a steady temperature through the year, typically around 10 to 12 °C. That consistency is the main reason ground source systems reach a higher seasonal efficiency than air source in cold weather: the source does not get colder just when the building needs the most heat. Efficiency is declared and compared as SCOP under BS EN 14825 using the test points in BS EN 14511, and the same rule applies as for any heat pump: the lower the flow temperature the emitters need, the better the result.

Ground loop options

• Vertical closed-loop boreholes: the usual choice for commercial sites. Pipework is lowered into drilled holes and grouted. It needs little surface area but does need a drilling rig and access.
• Horizontal loops: pipe laid in trenches at shallow depth. It needs a lot of land and is mainly practical where there is open ground and the project is smaller.
• Open-loop systems: groundwater is abstracted from one borehole and returned to another or discharged. Output can be high, but the aquifer must suit it, and abstraction and discharge are regulated.
• Energy piles and foundations: loop pipework cast into structural piles on new buildings, which saves separate drilling.

The thermal response test

For a commercial field the ground cannot be guessed. A thermal response test (TRT) is carried out on a test borehole. A known amount of heat is put into the loop for a period, usually a couple of days, while temperatures are logged. The result gives the ground’s effective thermal conductivity and the undisturbed ground temperature, which feed the borehole design software.

The TRT is what turns a rough estimate into a design. It decides how many boreholes are needed, how deep they go and how far apart they are spaced. Spacing matters because boreholes that are too close draw heat from the same volume of ground, and the field’s temperature drifts downward over the years. A sound design is modelled for decades of operation, not only the first winter.

Balancing heating and cooling

A ground field behaves best when the heat taken from it is roughly balanced by the heat returned. Buildings with both heating and cooling demand, such as offices, hotels, leisure centres and some hospitals, can reject summer heat back into the ground, which recharges the field and can allow cooling with very little compressor use. Buildings with heating only draw the ground steadily colder, so the field is sized more conservatively. This is why ground source tends to suit year-round buildings. The comparison with air source is set out in the guide to air source versus ground source for a commercial building.

Permits and checks before drilling

Drilling is an engineering and regulatory task, not a plumbing one. Typical checks include:

• British Geological Survey records: geology and existing borehole data help predict ground conditions, and new boreholes are recorded with the BGS.
• Coal Authority: in former coal-mining areas, drilling can need permission and a review of the mining records.
• Environment Agency: open-loop systems that abstract groundwater need an abstraction licence or permit, and discharge to ground or water can need consent. Closed-loop schemes in sensitive groundwater areas still need a conversation with the regulator.
• Services and contamination: buried services, made ground and contaminated land change the drilling method and cost.
• Planning and access: drilling rigs, compound space and spoil handling need agreement with the planning authority and neighbours.

Because these checks run in parallel with design, they are part of why a ground source project is measured in many months rather than weeks. Realistic timings are in the guide on how long a commercial heat pump installation takes.

Running cost and when it pays back

The higher seasonal efficiency of ground source means less electricity for the same heat. Whether that saving justifies the extra capital depends on the load. A building that needs heat for most of the year, such as a care home, hospital, hotel or leisure centre, has many hours over which the efficiency gain accumulates. A building with a short heating season has fewer hours to repay the borehole field.

Do not rely on a headline figure for payback. Model it from the building’s own consumption, the TRT result and the electricity tariff. The capital allowance treatment is the same as for other heat pump plant, usually the Annual Investment Allowance for integral features; see capital allowances on heat pumps and the cost guide. Grant schemes come and go, so confirm the status of any scheme on gov.uk before planning around it; the position is summarised on the grants and funding page.

Pre-design checklist

• Twelve months of heating and cooling consumption data
• A heat-loss calculation and the design flow temperature
• Available land for rig access and the borehole field
• Desk study of geology, mining records and groundwater sensitivity
• A thermal response test and long-term ground temperature modelling
• The DNO supply position for the added electrical load
• A commissioning plan that verifies the SCOP against design

Frequently asked questions

How does a commercial ground source heat pump work? A closed fluid loop collects heat from the ground and the heat pump lifts it to the temperature the building needs.

What is a thermal response test? A test borehole is heated and monitored to measure the ground’s thermal conductivity, which sizes the borehole field.

How many boreholes are needed? It depends on load, ground conductivity and space. Spacing and long-term ground temperature are part of the design.

Do boreholes need permits? Checks can include the BGS, the Coal Authority in mining areas and the Environment Agency for open-loop schemes.

Is ground source better than air source? It is usually more efficient and quieter but costs more and takes longer. See the guide to commercial air source heat pump sizing and siting for the other option.

For a view on your own building, use the form below for a desk feasibility; the reply comes by email.

For the wider differences from domestic systems, read commercial heat pump vs domestic heat pump.

For the full process, see the commercial heat pump installation overview.