Ground source heat pumps can work with ground loops, where a network of underground loops connects together and helps transfer heat into your home from the ground.
However, ground source heat pumps can also extract heat energy from underground sources through deep holes drilled vertically into the ground known as boreholes.
Boreholes differ from ground loops because ground loops are spread flat 1 – 2m underground across an outdoor space, whereas boreholes are drilled deep into the ground so the refrigerant containing system can be installed vertically rather than flat across the ground.
Boreholes can be beneficial for heat pump installation because they’re space efficient when compared to ground loops, requiring only a few feet of outdoor space, rather than the many square metres of space needed for ground loops, to install the collectors for ground source heat pumps.
Closed Loop Boreholes vs Open Loop Boreholes
There are different types of boreholes that can be drilled to help generate heat for ground source heat pumps.
Open loop ground source heat pumps are those that are open loop, so natural water sources are used as the transfer fluid so heat is extracted from the natural underground water source.
Closed loop ground source heat pumps are those that contain heat transfer fluid as part of a sealed system. The loop circles into the ground and back to the heat pump for the heat to be transferred out of the transfer fluid and into the heating system in your home. The transfer fluid then cycles back to the underground loop in the borehole to collect more heat.
In the closed loop system, the refrigerant fluid is pressurised, ensuring that the liquid flows effectively through the system.
How Deep are Boreholes for Heat Pumps?
Boreholes for heat pumps are from 60 metres to 200 metres deep. This is so they can be long enough to contain enough transfer fluid to adequately provide heat transfer to your heating system.
Fluid within the network in the heat pump is then pressurised to ensure that the flow capacity is suitable to flow into your property to the heat exchanger.
With sufficient pressure, ample refrigerant fluid and a heat pump in your home, you’ll be able to generating heating and hot water from the ambient heat stored underground, using only the power required to run your heat pump and the internal heat exchanger.
Can you install Boreholes Anywhere?
Boreholes can usually be installed pretty much anywhere, even in properties with small gardens or where a ground array would be impossible.
This is because Boreholes go deep into the ground rather than spreading out flat over a large area like a ground loop system would.
Boreholes require only a few mm between each hole, they don’t require a large amount of outdoor space and only 2 – 6 boreholes would be sufficient to provide enough heat for a family home.
Even terraced properties, with typically small gardens, could choose a ground source with boreholes as the heating system, rather than ground loops, provided there is suitable access to allow diggers and drill systems access to the space.
Surveys are generally conducted prior to the drilling of boreholes to make sure any holes drilled miss major subterranean structural networks like sewerage systems, water pipes, gas mains, tube stops or mine workings.
What types of Projects do Boreholes Suit?
For self build projects or upgrades from boilers to a ground source heat pump, loop arrays are generally recommended because they are much cheaper to lay and install when compared to drilling the equivalent number of required boreholes to provide adequate heat for the properties.
Simply put, it is cheaper to dig a series of 900mm to 1m wide trenches to a depth of 1 metre than it is to drill 1 to 6 holes to a depth of 60 to 90m. Drilling is expensive, more liable to go wrong and can lead to further complications.
For this reason, boreholes are generally only consider for;
- projects where shared boreholes make sense, like community upgrade schemes, district heating projects or large commercial properties with limited outside space.
- Boreholes can also be utilised where single properties are looking to install ground source heat pumps but lack sufficient outdoor space for loop array systems.
Most private installations would be better off considering ground loop systems rather than boreholes initially.
The Cost of Borehole Installation
Both types of ground source heat pump, those reliant on boreholes and those reliant on ground loops, which are also sometimes known as slinkies, because of their close visual resemblance to an uncoiled spring, is the cost of installation.
Ground works are expensive, so the required digging and drilling to install a ground source heat pump does increase the overall cost of choosing a heat pump for your property.
Boreholes are more expensive than ground array systems because ground arrays can be dug with an ordinary mini digger which will dig a trench around your outdoor space of between 2 – 5 metres deep and around 40 metres in length.
Boreholes require specialist drilling equipment, experienced drill operatives, known as drillers and ground surveying to make sure there’s nothing below the installation zone that will cause issues or prevent drilling successfully to the required depth.
Loop array vs Borehole Cost Comparison
The comparative cost of installation of boreholes vs slinky systems (or loop arrays) is shown below:
| Number of Bedrooms | Property Age | Borehole Price | Ground Loop Price | Borehole Price vs Ground Loop |
| 3 | pre 1965 | £12,389 | £9,639 | +28.52% |
| 5 | pre 1965 | £12,411 | £14,998 | +20.84% |
| 3 | 2002 – 2009 | £12,209 | £8,691 | +40.47% |
| 5 | 2002 – 2009 | £12,389 | £9,639 | +28.52% |
| 3 | 2009 – present | £12,209 | £8,691 | +40.47% |
| 5 | 2009 – present | £12,209 | £8,691 | +40.47% |
You can see that groundwork for boreholes is around 33.21% more costly than installing the equivalent system with a ground loop.
How do Boreholes work?
Boreholes work by pipping liquid through a network of pipes deep into the ground.
Once the holes have been drilled and sured up, cabling is added which includes a flow of liquid through sealed pipes in the holes.
The liquid, commonly a mix of water and antifreeze travels down through the pipe into the borehole where it gets heated by heat within the earth.
The pressurised system returns the heated fluid to your home where the heat exchanger in the heat pump extracts the heat from the fluid and uses it to heat hot water which flows around your home into heating systems and hot water tanks.
The cooled fluid now flows back around the closed loop system, into the boreholes and back into the ground to collect more heat.
Quite simply, boreholes are part of a ground source heat pump system and a vital feature for generating heat for your property.
Can boreholes go wrong?
Generally boreholes are a very reliable method of providing a route to heat stored in the ground. Boreholes can very often be “sured up” or “backfilled” after they’ve been drilled and the heat network system has been installed.
Backfilling of boreholes includes; filling the hole with sand or gravel to make sure they’re structurally sound and won’t collapse. However, you should be careful to consider what you use to backfill your boreholes with.
Both gravel and sand could affect the performance of your heating system because they’re both full of pockets of air. Air is a poor conductor of heat, so instead consider backfilling your boreholes with installer approved thermal grout.
Unlike gravel and sand, which can erode over time and which will contain lots of air, thermal grout is designed specifically to maintain optimum thermal conductivity, so will help to keep your heat pump system running efficiently.
Here’s how efficiency is measured for heat pumps using CoP
Other problems with boreholes can usually be resolved by the team you have installing them for you as they work. For example, if the ground is soft, flooded or erodes quickly then additional measure may be taken to help sure things up before your heat transfer pipes are installed.
Problems later on with boreholes are uncommon and most heat pumps are thoroughly tested by installers before being signed off as working.
What effects a boreholes conductivity?
The most affective elements for boreholes which can affect the performance of your heating system are;
- Geology and the thermal conductivity of the ground on which your property sits
- Spacing between boreholes – spaced too closely, boreholes may interfere with one another (i.e. space is recommended to allow boreholes to effectively gather heat and disperse cold without doing so directly into another borehole).
You’ll be able to get an understanding of the geology and thermal conductivity of the ground beneath your outdoor space with a survey. Once you have this, you’ll be able to calculate how many boreholes you need for the size of property you need.
Understanding the geology is also key to helping determine the spacing between your boreholes. Areas with crumbly, loose soil may need more holes spaced further apart than areas where soil is claggy and clay like.
Effective spacing of boreholes prevents boreholes from transferring heat and cold to each other and helps to prevent against borehole collapse.
How are boreholes installed?
Boreholes are generally installed using drilling technology to drill narrow, straight holes into the surface of the earth outside you’;re property.
This equipment is generally expensive which is one of the reasons why boreholes can be more expensive than loop array systems.
Usually a test hole is drilled first, this will be around 60m to 200m deep and is used to confirm the accuracy of any survey you have had to complete on the ground around your property prior to work beginning.
Once the test hole has been drilled and the geology of the earth has been confirmed, the rest of the holes will be drilled.
Pipework for the ground source heat pump is inserted and installed. Once the pipework is connected up, the pipework can be pressure tested to ensure that the pipework holds pressure consistently.
This pressure test is important for ensuring the heating fluid can flow down into the boreholes and up into the heat exchanger in your home.
Engineers will also test the thermal response of the boreholes which have been installed.
Finally, you heat pump, which will usually be installed inside your property can be installed and you’ll be able to start using the system.
Boreholes FAQs
When do you install boreholes for a ground source heat pump?
Boreholes are usually the first step of installation of a ground source heat pump. However, loop arrays are commonly more popular than boreholes for ground source heat pumps.
Boreholes tend to be; more expensive and more difficult to install than loop arrays.
Boreholes are commonly used for ground source heat pumps when;
- outside space is limited and there’s insufficient space for a loop array
- shared heat pump schemes and social housing projects where drilling can be done on public property like into roads
- where a water source is available nearby which could provide the opportunity for an open loop system
- for commercial properties who have heat requirements but limited external space
- if loop arrays are insufficient or impossible for any other reason
Do air source heat pumps need boreholes?
No, air source heat pumps don’t need boreholes or loop array systems because they extract heat from air around your property and use this heat to increase the temperature of water within your heating and hot water system. Air source heat pumps are a sealed unit which only require pipes to transfer heat from the unit into your home.