How many boreholes does a ground source heat pump need?

Most people who are considering investing in a ground source heat pump will end up doing a lot of research on the topic. It’s a considerable project to invest in, especially compared to a more standard gas boiler, and you need to know what you’re getting into. 

One of the most significant parts of these projects is the drilling of boreholes, and it’s important to know how many boreholes a given system will require, and the factors that this number is based on. 

The bare minimum

For a closed-loop geothermal system to work, you need to drill at least one borehole. Larger homes will require up to three boreholes, and larger commercial properties can require more still. 

All ground source heat pump installation projects start with an assessment process, during which the team involved will be able to determine exactly how many boreholes will be required for your property. 

Their calculations will generally be based on the following factors. 

Energy per borehole

To understand how many boreholes a specific property will need, you have to start with the heating requirements of the property, ideally calculated in kW during the coldest part of the year. 

Each individual borehole will be able to provide between 3kW and 5kW, depending on the specific geology of the site in question. Based on those figures, and the number of kW required to heat your property, you can quickly determine how many boreholes you’ll need. 

Geological considerations

The geological makeup of the ground that the boreholes are being drilled into will have an impact on thermal conductivity, thereby impacting how many boreholes are required for your project. 

More porous rock and soil types (such as gravel, sand, or highly fissured karst limestone) tend to be less efficient, while quartzite, sandstone, saturated granite and wetter clays and silts tend to facilitate much higher conductivity. 

Depth

Another variable involved in the number of boreholes required for a project is how deep they can be drilled. As boreholes put out an average of between 35 W/m and 50 W/m, the deeper the borehole, the more energy it will be able to provide. 

The standard depth for residential projects is around 100 metres, and while it is possible to dig deeper than that, it is generally more efficient cost-wise to just dig more boreholes.

Considerations

Vertical boreholes don’t take up a lot of space, but they do need to ideally be at least 5 metres apart, so that they don’t start to drain thermal energy from the same area and negatively impact overall efficiency. 

If your property doesn’t have sufficient space, there may be some clever workarounds (such as digging deeper boreholes), but you will need to consult a specialist heat pump installation service. 

Additional boreholes are expensive, but there are ways you can reduce the number required for any given building, such as by improving the insulation of your property. It’s worth having a survey done, at the very least, to see how easy or difficult your project in particular will likely be.