The ground array is the part of a ground source heat pump system that quietly decides whether the whole installation performs well. It is the buried pipework that collects heat from the ground and carries it back to the heat pump. If it is too small, badly spaced, poorly installed or placed in unsuitable ground, even a good heat pump can struggle.
For homeowners, the big questions are practical: do you have enough land, is a borehole better, how disruptive is the work, and what should the installer calculate before anything is dug? This guide explains the main types of ground array, how they are installed, and what to check before committing to a ground source heat pump.
Contents
- 1 Key Takeaways
- 2 What Is A Ground Array?
- 3 Horizontal Ground Arrays
- 4 Slinky Pipes Versus Straight Pipes
- 5 Vertical Borehole Arrays
- 6 Water Source Arrays
- 7 How Ground Arrays Are Sized
- 8 Ground Array Installation Process
- 9 Common Mistakes To Avoid
- 10 Case Study: Choosing Between Trenches And Boreholes
- 11 Expert Insights From Our Heating Engineers
- 12 Frequently Asked Questions
- 12.1 How Much Land Do You Need For A Ground Array?
- 12.2 How Deep Are Ground Source Heat Pump Trenches?
- 12.3 How Deep Are Ground Source Heat Pump Boreholes?
- 12.4 Are Slinky Pipes Better Than Straight Pipes?
- 12.5 Can A Ground Array Be Too Small?
- 12.6 Can You Install A Ground Array Under A Driveway?
- 12.7 Do You Need Planning Permission For A Ground Array?
- 12.8 How Long Does A Ground Array Last?
- 13 Summing Up
Key Takeaways
- A ground array must be designed around the property’s heat loss, ground conditions, available space and hot-water demand.
- Horizontal arrays usually cost less than boreholes but need more land and good access for excavation.
- Boreholes save garden space but need specialist drilling, suitable geology and a larger budget.
- An undersized ground array can reduce efficiency, increase running costs and make the heat pump rely too heavily on backup heating.
- Use an experienced designer or installer who can explain the calculations, not just the pipe layout.
What Is A Ground Array?
A ground array is a network of buried pipes connected to a ground source heat pump. A fluid, usually water mixed with antifreeze, circulates through the pipework and absorbs low-grade heat from the ground. The heat pump then upgrades that heat to warm the home and, where designed for it, the hot water cylinder.
The ground is not an unlimited battery. Heat is replenished by solar gain, rainfall and natural ground conditions, but the array still has to be sized correctly. If the system extracts heat faster than the ground can recover over the heating season, performance suffers.
That is why ground array design should follow a proper heat loss calculation. Guessing from floor area or copying a neighbour’s system is not good enough. Two similar-looking homes can have very different insulation, emitter temperatures and hot-water demand.
Horizontal Ground Arrays
Horizontal arrays are laid in trenches across a garden, field or plot. The pipes may be straight runs or coiled “slinky” loops. Which? notes that horizontal systems are commonly laid in shallow trenches around one to two metres deep, and that they need more ground area than vertical arrays.
Horizontal arrays are usually the most cost-effective option when there is enough land and machinery access. They avoid specialist drilling and can work well for rural homes, self-builds and larger gardens. The disruption is visible, though. Trenches have to be dug, soil stored, pipework laid, pressure tested and the land reinstated.
The ground should not be heavily shaded, waterlogged in a problematic way, built over without design approval, or planted later with deep-rooted trees that could affect access and pipework. Wet soil can transfer heat well, but drainage, compaction and reinstatement still need care.
Slinky Pipes Versus Straight Pipes
Slinky pipes are coiled loops laid in trenches. They can reduce trench length compared with straight pipe layouts, which may lower excavation work where land is available but not unlimited. Straight pipe arrays spread pipe more evenly and may be easier to model in some designs, but they often need more trenching.
Neither is automatically best. The right choice depends on heat demand, soil conditions, trench spacing, installer design approach and available land. Be wary of anyone who recommends a layout before completing the heat loss calculation and site assessment.
Vertical Borehole Arrays
Boreholes are vertical ground arrays drilled deep into the ground. They suit properties where horizontal land is limited, where the owner wants less surface disruption, or where a shared or larger system makes drilling more economical.
Competitor guidance from Kensa and GreenMatch commonly places domestic borehole depths in the broad range of about 50m to 200m, although exact depth depends on geology, heat load and design. Boreholes take up little surface width, but the drilling rig needs access, working space and suitable ground conditions.
Boreholes are usually more expensive than horizontal arrays for a single property. Their advantage is space efficiency and stable performance when designed properly. They are also attractive for shared ground loop schemes, larger homes and commercial projects.
Water Source Arrays
If a suitable lake, pond or watercourse is available, a water source collector can be very efficient because water transfers heat well. In practice, this route needs careful permissions, environmental checks, water depth, access and protection from damage.
A water source array is not simply a coil of pipe thrown into a pond. It needs secure installation, adequate volume and proper design to avoid freezing risk, ecological problems or poor heat transfer.
How Ground Arrays Are Sized
The starting point is the building’s heat loss. The installer should assess insulation, glazing, airtightness, floor area, room-by-room heat demand, hot water, radiator or underfloor heating output, and the flow temperatures the heat pump will need.
Then the ground side is designed. Soil type, moisture, available area, trench spacing, borehole depth, thermal conductivity and seasonal recovery all influence the result. A rule of thumb can be useful early in planning, but it should not replace design. Which? gives an example rule of thumb of around 10m of trench per kilowatt of heat pump size, but final design must be site-specific.
Ground Array Installation Process
The process usually starts with survey and design. The installer confirms heat demand, site access, services, drainage, boundaries, trees and possible constraints. For boreholes, drilling contractors may need geology information and access planning.
For horizontal arrays, machinery digs trenches, pipe is laid to the designed spacing, circuits are pressure tested and brought back to a manifold or plant-room connection. The trenches are then backfilled carefully to avoid damaging the pipework. The ground may take time to settle and recover visually.
For boreholes, drilling comes first. U-tubes are lowered into the boreholes, thermal grout is used where required, and the pipework is connected back to the heat pump. Borehole work should be handled by specialist contractors because poor drilling, grouting or manifold design is difficult to fix later.
Common Mistakes To Avoid
The most serious mistake is undersizing. An undersized array can lead to lower brine temperatures, poorer heat pump efficiency and increased use of backup heat. The system may still run, but it will not deliver the performance the homeowner expected.
Other mistakes include ignoring plant-room space, failing to allow access for future maintenance, placing pipework where future extensions or landscaping will conflict, and treating hot water as an afterthought. Ground source systems often need a suitable cylinder and enough indoor space for equipment.
It is also worth comparing ground source with air source heat pumps. If land or budget makes the ground array difficult, an air source system may be the better practical choice.
Case Study: Choosing Between Trenches And Boreholes
Background
A homeowner in a rural detached property wanted a ground source heat pump to replace oil heating. The property had a large garden but also mature trees and a planned garage extension.
Project Overview
The early assumption was that horizontal slinky trenches would be simplest. A site review showed that parts of the garden were unsuitable because of future building plans and tree protection zones.
Implementation
The design compared a reduced horizontal array with boreholes. Boreholes cost more upfront, but they avoided future conflicts and preserved more usable garden space. The final decision was made after heat loss, access and long-term landscaping were considered together.
Results
The project avoided a layout that would have looked cheaper on paper but caused problems later. The lesson is that the best ground array is not just the cheapest to dig. It is the one that suits the property for the next 25 years.
Expert Insights From Our Heating Engineers
One of our senior heating engineers with over 20 years of experience says the ground array should be treated as the heat source, not as a plumbing accessory. If the array is wrong, the heat pump spends its life trying to compensate.
He recommends asking the installer to explain the heat loss calculation, array sizing and ground assumptions in plain English. If they cannot explain why the array is the size it is, the design probably has not been thought through carefully enough.
Frequently Asked Questions
How Much Land Do You Need For A Ground Array?
It depends on the home’s heat demand, soil conditions and array type. Horizontal arrays need much more land than boreholes, so a large garden or field is often required. A proper heat loss calculation is the only reliable starting point.
How Deep Are Ground Source Heat Pump Trenches?
Horizontal trenches are commonly around one to two metres deep, but the exact depth and spacing depend on the design. The installer must also account for services, drainage, soil and future landscaping.
How Deep Are Ground Source Heat Pump Boreholes?
Domestic boreholes are often somewhere between about 50m and 200m deep, depending on geology and heat demand. Some larger or specialist projects may use different depths.
Are Slinky Pipes Better Than Straight Pipes?
Not always. Slinkies can reduce trench length, while straight pipes may spread extraction differently. The better option depends on the site and design calculation, not a universal rule.
Can A Ground Array Be Too Small?
Yes. An undersized array can reduce efficiency, lower output and increase backup heating use. It may also make the ground temperature around the array recover poorly through the heating season.
Can You Install A Ground Array Under A Driveway?
Sometimes, but it needs careful design. Built-over ground may behave differently, access for future works is harder, and pipe protection becomes more important. Do not assume it is acceptable without installer approval.
Do You Need Planning Permission For A Ground Array?
Many domestic ground source installations fall under permitted development, but rules and site constraints vary. Listed buildings, conservation areas, boreholes and water source systems may need extra checks.
How Long Does A Ground Array Last?
The buried pipework can last for decades when designed and installed correctly. It may outlast the indoor heat pump unit, which is why careful sizing and layout matter so much at the start.
Summing Up
Ground array installation is one of the most important decisions in a ground source heat pump project. Horizontal arrays can be cost-effective where land is available. Boreholes suit tighter sites but cost more and need specialist drilling. Water source arrays can work well on suitable properties, but they need careful permissions and design.
The right array is the one sized to the building, the ground and the household’s heat demand. Get that right and the heat pump has a stable source of energy for decades.
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