Ground Source Heat Pumps: Design, Slinky Coils, Boreholes, and MCS Certification

Ground source heat pumps (GSHPs) extract heat energy from the ground and deliver it as useful heat for space heating and domestic hot water. The ground remains at a relatively stable temperature year-round (around 8–12°C in the UK at shallow depths), providing a consistent heat source that makes GSHPs more efficient than air source heat pumps in cold weather. This guide covers how GSHPs work, ground loop options, system design, MCS certification, the Boiler Upgrade Scheme grant, and commissioning requirements.

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How Ground Source Heat Pumps Work

A ground source heat pump operates on the same refrigeration cycle as a domestic refrigerator, but in reverse — it moves heat from a low-temperature source (the ground) to a higher-temperature delivery system (your heating circuit and hot water cylinder).

The cycle has four stages:

1. Evaporation: A refrigerant fluid circulating in the ground loop absorbs heat from the ground. Even at 8°C, the ground contains enough heat energy to evaporate the refrigerant (which boils at a low temperature).
2. Compression: The compressor increases the pressure and temperature of the refrigerant vapour. This is the main energy input — the heat pump uses electricity to run the compressor.
3. Condensation: The hot, high-pressure refrigerant passes through the heat exchanger, transferring its heat into the heating circuit (the water that circulates to radiators and the hot water cylinder). The refrigerant condenses back to liquid.
4. Expansion: The refrigerant passes through an expansion valve, its pressure drops, its temperature falls, and it returns to the evaporator to collect more heat from the ground.

The efficiency of a heat pump is expressed as the Coefficient of Performance (COP) — the ratio of heat energy delivered to electrical energy consumed. A typical GSHP has a COP of 3.0–4.5, meaning it delivers 3–4.5 units of heat for every unit of electricity consumed. The Seasonal Performance Factor (SPF) accounts for real-world variation over a full year and is typically 3.0–4.0 for a well-installed GSHP.

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Ground Loop Types

Horizontal Slinky Ground Loops

The most cost-effective ground loop for properties with sufficient land area. Pipes are laid in trenches at 1.2–1.5m depth (below the frost line and where ground temperature is more stable) in a slinky (overlapping coil) pattern. The slinky pattern allows more pipe length in a given trench length — a key advantage where trench digging is the primary cost.

Typical slinky dimensions:

• Coil diameter: 900mm
• Coil pitch: 300–500mm (overlapping coils)
• Pipe: 40mm or 32mm HDPE (high-density polyethylene), pressure-rated PN10 or PN16
• Effective pipe length per metre of trench: approximately 3× the trench length

Ground area required: approximately 15–25m² of trench per kW of heat pump output (variable by soil type — clay soils have higher thermal conductivity than sandy or gravelly soils). A 10kW GSHP would typically require 150–250m² of trench area. The ground above the trench cannot be paved, built on, or planted with deep-rooting trees — it must remain as lawn or shallow-rooted planting to allow the ground to recover its heat store via solar radiation and rainwater percolation.

Vertical Borehole Ground Loops

Where land area is limited, boreholes provide access to deeper ground at a stable temperature. Boreholes are drilled vertically to depths of 50–150m and grouted with a thermally conductive grout to ensure good thermal contact between the pipe and the surrounding geology. Deep ground temperature is typically 10–13°C and very stable year-round.

Typical borehole parameters:

• Borehole diameter: 130–200mm
• Depth: 50–150m per borehole
• Pipe: 32mm or 40mm U-tube HDPE loop within each borehole
• Heat yield: approximately 50–70 W/m of depth (in average UK geology — igneous rock yields more, clay and chalk less)
• Boreholes required: calculated from heat pump output and available heat yield per metre

Borehole drilling requires specialist equipment and contractors and is significantly more expensive than horizontal slinky systems. However, vertical boreholes are the preferred option in urban plots and on properties without sufficient garden area. Multiple boreholes must be spaced at least 5–8m apart to prevent thermal interference.

Horizontal Ground Loop (Straight Runs)

In addition to slinky coils, ground loops can be laid as straight parallel runs at 0.5–1.0m separation in wide, shallow trenches. This approach uses single-length pipes (rather than pre-coiled slinky) and requires more trench length but is simpler to install in suitable ground. Required depth is 1.2m minimum. Straight runs suit wide, flat sites where trenching equipment can work efficiently.

Surface Water and Groundwater Systems

Some sites have access to a lake, river, or groundwater source that can be used instead of a ground loop:

• Closed-loop lake/pond system: Coils of HDPE pipe laid on the bed of a lake or large pond. The water body acts as the heat source. Requires the water body to be of sufficient volume to recover its heat store — a rule of thumb is at least 1,000m³ water volume per kW of heat pump output. Environmental permits are required.
• Open-loop groundwater: Groundwater is extracted from a supply borehole and passed through a heat exchanger (or directly through the heat pump evaporator), then returned to a soakaway or discharge borehole. High COP performance (groundwater is typically 10–12°C constant), but requires a groundwater abstraction licence from the Environment Agency and a return permit.

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Ground Loop Design and Sizing

Ground loop sizing must be carried out by a suitably qualified designer. Key inputs:

• Heat pump output: Determined by a full heat loss calculation for the property (to EN 12831) — not a rule-of-thumb estimate. The heat pump must be sized to meet the design heat load at the design outdoor temperature (e.g., -3°C in most of England) at the maximum permitted flow temperature (typically 45–50°C for radiator systems, or 35°C for underfloor heating).
• Ground thermal conductivity: Varies significantly by soil/rock type. Clay: 1.0–2.0 W/mK; limestone: 2.0–3.0 W/mK; granite: 2.5–4.0 W/mK. A thermal response test (TRT) is the most accurate way to determine borehole thermal conductivity for larger systems (usually required for >3 boreholes or commercial projects).
• Annual heat demand: The ground loop must be sized to supply heat over a full year without the ground temperature declining over successive years (ground regeneration). Software such as EED (Earth Energy Designer) or GLD (Ground Loop Design) is used for this.

Undersizing the ground loop is a common cause of poor GSHP performance — the ground temperature drops below design conditions and the heat pump COP falls, or the unit goes into defrost mode or shuts down.

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Heat Distribution System

GSHPs work most efficiently when the delivery temperature is low. This means the heat distribution system must be designed for low-temperature output:

Underfloor Heating (UFH)

The ideal distribution system for a GSHP. UFH operates at 35–45°C flow temperature, which is achievable from a GSHP with a COP of 4.0+ in heating-only mode. The low surface temperature and large emitter area of the floor provide excellent heat transfer. See: Underfloor Heating: Wet UFH, Pipe Layout, and Manifolds (#25).

Radiators

Existing radiators designed for 80°C flow temperature (old gas boiler standards) will not perform adequately at 45–50°C. The radiator output at 45°C is approximately 50% of its rated output at 80°C. To use existing radiators with a GSHP, they must be oversized (replaced with larger models) — typically by a factor of 1.5–2×. In practice, a property changing from gas boiler heating to a GSHP should have a radiator survey and replacement as part of the installation.

See: Radiator Sizing: BTU Calculation and Panel Type Guide (#37).

Fan Coil Units

In commercial and some residential applications, fan coil units (FCUs) connected to low-temperature hot water circuits provide rapid heat-up and can operate effectively at 45°C. FCUs are common in renovation projects where UFH is not possible and radiator replacement is impractical.

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Domestic Hot Water with a GSHP

GSHPs can provide domestic hot water, but this requires the heat pump to produce higher temperatures (55–60°C for legionella prevention) than for space heating. At higher temperature lift, the heat pump COP drops. Options:

• DHW from heat pump with legionella cycle: The heat pump serves a large hot water cylinder (typically 200–300 litres for domestic). The cylinder is heated to 50–55°C by the heat pump. A timed legionella pasteurisation cycle heats the cylinder to 60°C once weekly — this may be provided by an immersion heater element within the cylinder.
• Separate solar thermal for DHW: On larger properties, a solar thermal collector pre-heats cold water before it enters the cylinder, reducing the temperature lift required from the heat pump and improving overall system efficiency. See: Solar Thermal Systems (#71).
• Buffer vessel: Some GSHP installations include a buffer vessel between the heat pump and the heating circuit to prevent short-cycling and provide hydraulic separation.

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MCS Certification

To access the Boiler Upgrade Scheme (BUS) grant and most other government incentives, GSHP installations must be carried out by an MCS (Microgeneration Certification Scheme) certified installer. MCS certification applies to both the installation company and the specific product installed:

• The heat pump manufacturer and model must be on the MCS product register
• The installation company must hold a current MCS installation certificate for ground source heat pumps
• The system must be designed and installed to MCS 003 (Heat Pump — Ground Source) standard and commissioned in accordance with the standard
• A commissioning record and MCS certificate must be issued to the customer and registered on the MCS database

Without MCS certification, the customer cannot claim the BUS grant and the installation may not be recognised for EPC purposes.

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Boiler Upgrade Scheme (BUS) Grant

The UK Government's Boiler Upgrade Scheme (BUS) provides grants toward the cost of eligible heat pump installations to replace fossil fuel heating systems in eligible dwellings. For ground source heat pumps:

• Grant value (2024–2025): £7,500 per property
• Eligibility: Property must have a valid EPC with no outstanding recommendations for loft insulation or cavity wall insulation (these must be completed first). The property must be in England or Wales. Flats and off-gas-grid properties using oil heating are also eligible.
• Application: The MCS-certified installer applies on behalf of the customer. The grant is paid to the installer, who reduces the customer's invoice by the grant amount.

The BUS is scheduled to run until 2028 and has a budget cap — check the current status and grant levels before quoting customers. See also: Air Source Heat Pumps: BUS Grant (#58).

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Installation Considerations

Planning Permission

Horizontal ground loop installation (trenching) is normally permitted development in England for domestic properties. Vertical borehole drilling may require planning permission in some areas — check with the local authority. Boreholes may also require an environmental permit from the Environment Agency depending on depth and location.

Groundworks and Trenching

For horizontal systems, excavation is the major cost. Mini-digger trenching is the most cost-effective method for most domestic gardens. Trench depth: 1.2–1.5m. After pipe installation, the trench is backfilled with sand or fine soil to ensure good pipe-to-ground thermal contact. The surface is reinstated and will be indistinguishable from the surrounding garden within a growing season.

Pipe Material and Pressure Rating

Ground loop pipework uses HDPE (high-density polyethylene) to PN10 or PN16 pressure rating. Standard sizes for domestic systems are 32mm or 40mm. HDPE is jointed using electrofusion welding for below-ground connections — compression or push-fit fittings are not approved for buried GSHP ground loops. All below-ground joints must be pressure-tested before backfilling.

Antifreeze

The ground loop fluid (primary fluid) is a water/antifreeze mixture — typically 25% monoethylene glycol (MEG) or monopropylene glycol (MPG) to protect against freezing to -15°C. MPG is food-safe and preferred where groundwater contamination is a concern. The antifreeze concentration must be checked and corrected at commissioning and at annual service. The primary fluid circuit is closed and separate from the heating circuit via the heat pump's internal heat exchanger.

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Commissioning a GSHP

GSHP commissioning must follow the MCS 003 procedure and be carried out by the MCS-certified installer:

1. Ground loop pressure test: Test the ground loop circuit to 1.5× working pressure (typically 4.5–6 bar) for 30 minutes minimum. Any pressure drop indicates a leak — locate and repair before proceeding.
2. Ground loop flushing: Flush the primary circuit to remove debris and dissolved air. Fill with antifreeze solution to the specified concentration.
3. Heating circuit commissioning: Flush and fill the secondary circuit (heating and hot water). Pressurise to design pressure. Balance the distribution circuit (radiators or UFH manifold).
4. Heat pump start-up: Follow the manufacturer's commissioning procedure. Allow the heat pump to reach steady state before recording performance data.
5. Performance verification: Record flow and return temperatures on the ground loop (primary) and heating circuit (secondary). Calculate instantaneous COP. Compare against design targets.
6. Controls commissioning: Programme the time schedule, set target temperatures, and configure any smart controls or demand-side management settings.
7. MCS registration: Complete the MCS commissioning record and register the installation on the MCS database. Issue the customer with their MCS certificate (required for BUS grant application).

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Maintenance

GSHPs require less routine maintenance than gas or oil boilers — there are no combustion components and no flue. Annual service should include:

• Check antifreeze concentration in ground loop fluid (use refractometer)
• Check ground loop pressure — if pressure has dropped, investigate for leaks
• Check heating circuit pressure, expansion vessel, and relief valve
• Check refrigerant circuit — examine for leaks (visual + electronic detector for F-gas compliance)
• Check filter strainers on both circuits
• Verify controls operation — thermostat, programmer, zone valves
• Record SPF data where a heat meter is installed

Ground source heat pumps have a design life of 20–25 years for the heat pump unit and potentially 50+ years for the ground loop.