What are boreholes, and how are they used for ground source heat pumps?

Boreholes are vertical holes the width of a dinner plate that are drilled into the ground to bring thermal energy up to a ground source heat pump. They save space and minimise disruption, while only needing a small amount of space at ground level for the number of homes they serve.

What properties are boreholes for ground source heat pumps best suited to?

The beauty of boreholes is that they can be installed at almost any property, even ones with little outdoor space. Drill rigs come in a variety of sizes, and can even be transported through homes to get to the desired drilling location. They need only 150 mm of lateral space at ground level and are completely hidden once installed. Typically, they are drilled to a depth of between about 80 and 300 metres. Boreholes are fitted with a loop of PE100 pipe encased in thermal grout. Of course, enough space and access is needed for a drill rig to get in, but these vary in size depending on application. A small rig serving a single home can be smaller than a car parking space, whereas a large rig serving new build sites can be higher than the highest diving board at an Olympic swimming pool.

Boreholes are ideal for What are Networked Ground Source Heat Pumps?Networked Heat Pumpsnetworked ground source heat pump schemes, typical in New Build DevelopmentsHousing Developmentsnew build developments and Social HousingSocial Housingsocial housing estate retrofits. They are also the recommended option in large commercial projects with high heat demands and smaller sites where saving space is a priority.

How much do boreholes cost?

While boreholes are more expensive to install than What are Slinkies & Straight Pipes?What are Slinkies & Straight Pipes?slinky pipes – because they require specialist drilling – they become cost-effective on networked heat pump schemes for estates, large commercial projects and any other projects with higher heat loads. Once installed, boreholes provide efficient, discreet, low-maintenance thermal energy to ground source heat pumps. Ask Kensa for cost guidance tailored to your site.

How many boreholes do I need for my property?

The number of boreholes you need will depend on your required heat load. An 80-to-100 metre borehole typically provides 3 to 5 kilowatts of extractable heat, in many cases enough to serve a single property. A deeper borehole can often serve multiple properties. For larger projects, such as a commercial or multi-home scheme, a thermal response test and geothermal design is needed to determine the number needed.

How do boreholes for ground source heat pumps work?

Boreholes are drilled by rotating and advancing a drill bit into the ground, removing soil and rock cuttings to create a stable hole for installing the borehole probe. A simple u-shaped piece of ground loop pipework is fitted into the borehole down to its depth, where the temperature is stable all year round. At ground level the ground loop is connected to the ground source heat pump via a lateral trench and manifolds. The space around the ground loop in the borehole is backfilled with a thermal grout to ensure thermal conductivity from the ground to the borehole pipe.  Fill, flush and purge operations are carried out by filling the borehole with clean water, flushing through any debris or contaminants, and purging the system. Then thermal transfer fluid is added to the pipes. The drilling contractor will perform a pressure test, cap the plastic ground array pipe and issue a certificate before leaving site.

Once installed, the ground source heat pump pushes the thermal transfer fluid through the closed loop system, which warms up when it circulates around the u shape in the ground and transfers heat back into the pump. Then the process is repeated.

Do ground conditions affect the performance of boreholes?

A borehole’s performance depends on the geology of your site, since some rock types conduct heat better than others. A borehole in granite extracts twice or three times as much heat as one in loose stones, for example. So it is important to know what you are drilling into. Kensa can help you understand your ground conditions. This involves assessing existing data on ground conductivity and any potential obstructions, and if necessary, Thermal Conductivity Tests. These are vital aspects that inform the design of the borehole network, often referred to as the ground array.

Do I need permits to drill boreholes?

Before drilling boreholes, you may need to obtain some simple permits, though not always. In some cases you may need to get a permit from the Environment Agency and/or Mining Remediation Authority if the site is located in a reporting area. You can check what they are on the UK government website. Since ground source heat pumps are generally considered permitted development, you will not need planning permission in most cases.

How much maintenance do boreholes need?

Boreholes require very little maintenance. In most cases they are ‘fit and forget’. When issues do occur, they can be dealt with at ground level with no need for further drilling. As with any heating system, heat pump units themselves benefit from checks over the course of their lifetime, to ensure everything is working properly.