The single most common question we are asked before a heat pump installation is which process will be necessary; borehole drilling or horizontal ground array? The following breakdown should help with understanding the differences.
Boreholes are commonly used in groundworks for heat pumps, particularly when space is limited. We have access to some very compact drilling equipment so access is rarely a problem.
Our very experienced team will take into account the heat loss of the property and geology of the ground to specify the meters of boreholes required. If there is any doubt of ground type, we undertake a thermogeological survey to confirm the right specification for your project, so you can be confident that the proposed heat pump is the right one for your home.
Depending on the ground type boreholes often extract between 35 - 50 Watts per meter making this a very efficient and ultimately cost-effective option.
Our boreholes are drilled by very experience local drillers who know Cornwall and the ground beneath very well. Once the borehole is drilled, polyurethane pipe is lowered into in with a U-bend at the bottom. When the pipe is in place the borehole can be back filled with Bentonite which can improve the thermal conductivity. In some cases the soil extracted from a borehole may have very good thermal conductivity and be reused instead of Bentonite, this decision would need to be carefully considered.
1. Design of boreholes and geographical survey
This is a very important part of the process and a lot care must be taken to make sure you have:
2. Borehole drilling, installation of collector (pipework) and back-filling
3. Trench work from boreholes to the subterranean manifold chamber
4. Pipework from the borehole to the manifold and any fusion welding that is needed
5. Trench work from the manifold to the plant room where the ground source heat pump is to be situated
6. Pipework from the manifold to the plant room and insulation
7. Fill pipework with water and anti-freeze solution
As an alternative to boreholes and if you have some land, it is worth considering a horizontal ground array. As specialist drilling equipment is not required, a horizontal ground array can costs less than a borehole.
There is however, a need for much more space, because the extraction rate is less at around 20W per meter on the same ground that you would expect 40-50 Watts per meter from a borehole.
There are a few very important points to remember when installing a horizontal ground array:
1. Trench work in a series of straight trenches linked to subterranean manifold
2. Pipework (collectors) to manifold incl. fusion welding, if required, and back-filling.
3. Trench work from the manifold to the plant room where the ground source heat pump is to be situated
4. Flow and return pipework from the manifold to the plant room.
Whether you have a trench or a borehole, the pipes will lead back to a manifold which can either be in a chamber in the ground or on the side of an external wall. If using a subterranean manifold, then the chamber must have a lid so that access is available at all times.
From the manifold into the property there will be 2 pipes (flow and return), which will be insulated. The reason for this is that it is best to keep as much pipework as active as possible, which means that it is able to continuously extract heat from the ground. When boreholes or horizontal trenches are used the pipework meets at the manifold, it is important that this pipework is not included as a collector and must be subtracted from the design.
These collectors can either come into the house above ground level, or in the case of a new build property, 6" underground waste pipe can be specified at the design stage. This way no pipework is seen entering externally because access to the plant room would be underground and up through the slab into the plant room.
The installation location is previously agreed with the customer in accordance with manufacturers’ instructions.
After a good installation of your ground source heat pump there is very little if any maintenance needed. There is often a yearly visit necessary to keep the guarantee valid and various manufacturers have yearly checks that they want a record of. The specific requirement will be explained as part of the handover depending on your choice of GSHP.