How many boreholes will I need?  How deep?

Here is a very approximate way to find out (Please do however, consult your heat pump supplier before making any decisions!)

The use of borehole collectors for GSHPs is necessary when there is insufficient ground area, or limitations on the use of existing ground areas e.g. valuable gardens etc.

Vertical borehole heat collectors usually operate at a higher efficiency, i.e. produce a higher heat yield per metre, than horizontal collectors. They are however, usually more expensive to install than horizontal collectors.

Vertical, i.e. borehole, collectors can provide between 25 (e.g. Clay) and 60 watts (saturated stone) per metre of active collector. This figure depends upon the thermal conductivity of the strata in which the borehole is drilled.

This is one of the reasons that we ask for a geological (BGS) survey of the proposed site. They will give a, usually reliable, projection for this value where you are. Relying on assumptions for this figure, can turn out to be expensive if your new heat pump installation fails to deliver the performance that you expected (and paid for!).

Should this data simply not be available, we usually work on a slightly conservative assumption of 45 w/m. The larger your installation, i.e. the more geothermal boreholes you require, the more important it is to get a correct value (BGS Survey).

The term active collector, is used to denote that part of the geothermal probe which is in direct contact with the ground source, i.e. the high thermal conductivity grout filling in the borehole.

The following calculation can be employed using an average value of 45W/m

Heat Pump capacity in KW 10KW (Heating, average family 3-4 bed house with reasonable insulation)

Assumed COP, ie, coefficient of performance of 4, i.e. for every 1KW input, 4Kw of heat are output.
Energy to be provided by ground source = 10 / 4 * 3 = 7.5 KW (7500W)
Energy yield per metre of active collector 45 W

Collector length required is therefore 7500/45 = 166.7 m

Please note, this can vary between a minimum of 136m and a maximum of 250m depending on the ground characteristics as referred to above

(This can be one borehole of 166.7 metres or two boreholes of approximately 85m, the most usual solution.)

Beware of heat pump salesmen who minimise this value and thus the project cost in order to make a sale!

How far apart?

Boreholes should be spaced at a minimum of 6m apart to avoid thermal linkage (and efficiency loss) between holes.

This value can also vary according to the ground characteristics. 6m is a good conservative average

How far from buildings?

A minimum of 3m to avoid the remote possibility of freezing ground temperature induced subsidence.

Also, remember to avoid siting boreholes anywhere near large trees whose subsequent root growth may damage the loops.

For a geothermal borehole quote, or advice, please call 01 491 828 208, or email info@synergyboreholes.co.uk

The information provided above is for general guidance purposes only and should not be used to determine individual borehole installation depths. Always consult your heat pump provider/installer.

Our apologies for the repeated use of the titles water boreholes and geothermal boreholes on these pages; such are the dictates of search engine requirements.

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