Ground Source Heat Pump Installation: Horizontal Collectors, Boreholes, Loop Sizing and Groundworks Requirements

Summary

Ground source heat pumps offer higher efficiency than air source because the ground maintains a stable temperature (10–12°C year-round in the UK) compared to air, which varies between -5°C in winter and 25°C in summer. However, the groundworks requirement makes GSHP installations significantly more expensive and disruptive than ASHP.

For groundworkers and heating engineers, GSHP installations are collaborative projects: the heating engineer designs and installs the heat pump, cylinder, and hydraulic system; the groundworker installs the ground collector (trenches for horizontal loops, boreholes for vertical loops). Both parties need to understand the interface between the ground loop and the indoor system.

Key Facts

• Horizontal collector (slinky loop) — PE-X or PE-RT flexible pipe buried in trenches at 1.0–1.5m depth; coiled (slinky) to increase pipe length per metre of trench; requires large land area
• Horizontal straight collector — parallel straight runs of pipe at 1.0–1.5m depth; lower pipe density than slinky; requires more land
• Vertical borehole — 80–150m deep boreholes containing a hairpin of PE pipe (U-tube); grouted with thermally enhanced grout; multiple boreholes for larger systems
• Ground array design — horizontal: approximately 30–60W/m of pipe extraction rate (depends on soil type: clay soaks up more than sandy; saturated soils perform best); vertical: approximately 50–80W/m of borehole
• Glycol mix — the ground loop is filled with a mixture of water and antifreeze (propylene glycol, ~25–30%); prevents freeze in UK conditions where ground temperature at 1m depth can reach 5°C in winter; see Environment Agency guidance for glycol use in ground loops
• EA abstraction licence — extracting heat from ground or groundwater may require an abstraction licence from the Environment Agency (EA) where the annual extraction exceeds a threshold; most domestic horizontal and borehole systems are below the threshold; confirm with the EA
• Borehole drilling contractor — vertical boreholes require a specialist drilling rig and a Ground Source Heat Pump Association (GSHPA) member contractor; not a standard groundworks task
• Building Regulations Part P — the electrical installation for the indoor GSHP unit is notifiable under Part P
• MCS 007 and MCS GS001 — MCS certification for GSHP; similar to ASHP certification but with additional ground loop design requirements
• Specific extraction rate — the heat extracted per metre of ground loop or per metre of borehole depth; depends on soil thermal conductivity; measured or estimated from soil type
• Thermal Response Test (TRT) — a test of a borehole to measure the in-situ ground thermal conductivity; used on larger commercial borehole installations to confirm design assumptions
• GSHPA (Ground Source Heat Pump Association) — the UK trade association for GSHP; provides design guidance (GSHPA Heat Pump Slinky Collector Design Guide); contractor accreditation scheme

Quick Reference Table: Ground Loop Sizing Rules of Thumb

These are indicative figures. Proper ground loop sizing requires site-specific soil assessment.

Detailed Guidance

Site Assessment for Ground Loop

Horizontal collector suitability:

• Available land area: minimum ~8–10× the heated floor area of the building; a 150m² house needs approximately 1,200–1,500m² of garden
• The land must be free of trees (tree roots can damage the pipe), underground services, and drainage features (French drains, soakaways)
• Soil type assessment: probe to confirm soil character at 1.5m depth; clay retains more heat than sandy soils; rocky ground may prevent trenching at required depth
• The land should not be heavily landscaped (removal of paving, patios, or outbuildings for trench access adds cost)

Vertical borehole suitability:

• Suitable where land area is insufficient for horizontal collectors
• The drilling rig requires vehicle access to the borehole location (typically 5m × 10m clear access for a truck-mounted rig)
• Geological assessment: confirm the borehole is likely to be drillable to the required depth without encountering unstable formations; consult a GSHPA-accredited drilling contractor for a preliminary borehole assessment
• Planning: boreholes typically do not require planning permission for domestic installations, but some local authorities require notification; check locally

Horizontal Ground Loop Installation

Trench specification:

• Depth: 1.2–1.5m minimum (below typical frost depth; below the zone of seasonal temperature variation)
• Width: typically 600mm for slinky loops; allows two slinky runs per trench
• Trench spacing: minimum 5m between trench centrelines to prevent thermal interference between loops
• Trench length: depends on total pipe required; slinky loops are typically 0.8–1.0m of pipe per metre of trench

Pipe specification:

• PE100 or PE-X pipe (typically SDR 11, rated to 6 bar)
• For slinky: purpose-made pre-coiled pipe (1m coil pitch is standard)
• Connections must be PE pipe-to-pipe heat fusion (electrofusion or butt fusion); push-fit or compression fittings are not acceptable underground (risk of long-term failure)

Backfill:

• Backfill around the pipe with fine material (no stones >20mm that could damage the pipe)
• Compact in layers; avoid direct compaction on the pipe
• Permanently mark the trench line at the surface (duck-board markers or surface markers) so future excavation does not damage the pipe

Manifold connections: The individual loop headers (flow and return from each trench) connect to a ground loop manifold in a manifold chamber near the house. The chamber must be accessible for flow balancing valves on each loop.

Pressure test: Before backfilling (and after), pressure test the ground loop to 1.5× operating pressure (minimum 6 bar) and hold for 60 minutes. Pressure drop indicates a pipe joint leak. Inspect all connections before backfill.

Glycol fill: After pressure testing, fill the ground loop with the water/glycol mix (typically 25% propylene glycol by volume — sufficient for -15°C freeze protection in UK ground temperatures). Connect to the indoor heat pump via insulated flow and return headers.

Vertical Borehole Installation

Vertical boreholes require specialist drilling equipment and should only be designed and installed by GSHPA-accredited contractors. The groundworker's role in a borehole installation is typically:

• Excavating the connection trench from the borehole head to the manifold/building
• Providing a hard-standing area for the drilling rig
• Preparing the manifold chamber

Borehole construction:

• Drilled by a specialist rig to the design depth (typically 80–150m)
• A PE hairpin (U-tube) is lowered into the borehole
• The borehole is backfilled with thermally enhanced grout (thermal conductivity >1.0 W/mK; typically a mixture of cement and bentonite with thermal additives)
• The grout fills the annulus between the pipe and the borehole wall; thermally connects the pipe to the surrounding rock; prevents groundwater contamination between strata

Multiple boreholes: Larger domestic or commercial GSHP systems may require 2–6 boreholes. Minimum borehole spacing is typically 5–8m to prevent long-term thermal depletion of the ground around the bore field.

Indoor GSHP System

The indoor GSHP unit is similar to an ASHP in its hydraulic connections: flow and return to the heating circuit, a DHW cylinder connection, and electrical supply. Key differences:

• The ground loop connection: flow and return from the ground loop manifold to the heat pump's ground-side heat exchanger; ground loop pump (brine pump) within the unit circulates the glycol/water mix
• A buffer tank is more commonly specified for GSHP than ASHP (the ground loop has higher thermal inertia; buffering reduces short-cycling)
• The flow temperature capability of GSHP is typically higher than ASHP at low outdoor temperatures (because the ground temperature doesn't drop as air temperature does); GSHP can maintain efficient operation on the coldest UK winter days

Legionella and DHW

Identical requirements to ASHP: DHW cylinder to be sized for the property (typically 200–300L for a domestic property); set point 50–55°C for normal operation; weekly pasteurisation to 60°C. A dedicated GSHP-compatible unvented cylinder with a direct coil (for higher flow temperatures) is the standard specification.

Frequently Asked Questions

Can I use an existing garden to install horizontal collectors after landscaping is complete?

Yes, but it is significantly more disruptive and expensive than installing collectors in an unlevel or pre-landscaped garden. Mature lawns can be reinstated; paving or patios must be removed, stored, and reinstated. Allow for this in the installation cost estimate. The additional cost may tip the economics in favour of vertical boreholes for landscaped gardens.

Can the ground loop be installed under a driveway or hardstanding?

Yes, but with caution. Ground loops under impermeable hardstandings (tarmac, concrete) can experience reduced performance because the hardstanding prevents rain from replenishing soil moisture. The loop under a hardstanding can also be harder to repair if a pipe fails. Where unavoidable, use pressure-class rated pipe and test rigorously before covering.

Do I need an Environment Agency licence for a domestic ground loop?

For most domestic ground source heat pump ground loops (horizontal or borehole), no abstraction licence is required. The de minimis threshold exempts most domestic GSHP ground loops. However, open-loop GSHP systems (which actually pump groundwater, use its heat, and return it to a different point) do require an abstraction licence. Confirm with the EA for any open-loop or water source heat pump installation.

Regulations & Standards

• MCS 007 / MCS GS001 — Ground source heat pump installation standard

• GSHPA Design Guide — Ground Source Heat Pump Association's design guidance for slinky and borehole systems

• BS EN 805 — Water supply; relevant to glycol mix and ground loop pipe specification

• Water Industry Act 1991 — relevant to borehole groundwater interactions

• Building Regulations Part P — electrical installation for indoor GSHP unit

• GSHPA — gshp.org.uk — design guides and contractor accreditation

• MCS 007 ground source guidance — mcscertified.com — MCS GSHP standard

• Environment Agency: groundwater abstraction guidance — EA guidance on GSHP abstractions

• air source heat pump installation — ASHP installation for comparison

• heat pump sizing heat loss — heat loss calculation for GSHP sizing

• mcs 007 heat pump standard — MCS certification and BUS grant eligibility

• heat pump cylinder sizing — DHW cylinder sizing for GSHP systems