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Borehole (Array) Dimensioning

The importance of correct GSHP and associated borehole dimensioning cannot be overstated. It is not unheard of for salesmen to "underestimate" the dimensions of borehole requirements in order to make a quick heat pump sale. Invariably, it is the end user that pays the cost for this in the long term, simply because the energy savings that were predicted, fail to materialise.

Get the procedures/documentation wrong the installation will not qualify for government support

Get the design/dimensions wrong  the reason for getting the GSHP i.e. energy/cost economy will simply disappear.

The comparison with a conventional oil/gas boiler can be used to illustrate this point in the following scenarios.

Scenario 1
Q What happens if the oil/gas boiler you install in your house is too small?
A The house does not get warm enough, The boiler runs continuously. In the extreme, up to 365 days a year, using gas from the mains or delivered oil, an expensive, but practically inexhaustible supply

Scenario 2
Q What happens if the Ground Source Heat Pump/Borehole system you install in your house is too small?
A The house does not get warm enough, the heat pump runs continuously, if it has a back up electric immersion heater, this will kick in (very expensively). The system will try to extract more and more heat from the ground around the boreholes. This will lower the ground temperature. 
The heat energy in the ground close to the borehole loops becomes exhausted. This in turn makes it harder (more expensive) for the pump to extract more heat. A vicious circle is formed. The heat pump starts to operate outside its comfort zone, it loses efficiency, the savings you expected, or were promised, do not materialise. Ultimately the system may fail.

Scenario 3
Q What happens if the GSHP is correctly dimensioned, but the associated boreholes are underdimensioned, i.e. too few and/or too shallow?
A Similarly to Scenario 2, the ground will become overcooled as the GSHP overworks the inadequate borehole length, a similar vicious circle forms.

Scenario 4
What happens if the GSHP is correctly dimensioned, but the associated boreholes are overdimensioned?
A The system will perform perfectly at full capacity, for a longer period. (perhaps longer than you require) There may therefore be an element of wasted money in the borehole installation.

Scenario 5
What happens if the GSHP is oversized, and the associated boreholes are correspondingly overdimensioned?
A The system will stop and start too often. Like a large car that is used for urban shopping trips, the "m.p.g." or efficiency will not be optimal. There will therefore be an element of wasted money in the installation.

Simply put, an oil or gas boiler can draw upon a practically inexhaustable energy supply via delivered oil or mains gas.

A GSHP can only extract the heat available in the ground which is only very slowly replenished. It is therefore essential not to "overdo it" by trying to extract heat over too long a period or from too small an area/volume of earth.

If longer run times are required, more boreholes are needed.

In different parts of the country the heat extraction/replenishment rates are affected by the differing geological strata. Thermal Conductivity, the scientific name for this property, can vary by a ratio of up to 3:1

This is where the correct, geology based, dimensioning of boreholes and borehole arrays has a critical role to play.

From the above it can be seen that two parameters and one characteristic are of great importance to the long term efficiency of a GSHP/borehole system.

The Parameters are

  • TEMPERATURES, more accurately temperature differences

  • TIME, i.e. heating periods, running times/durations

The Characteristic is: GEOLOGY, more accurately, thermal conductivity properties of the geology

These essential elements are reflected in the new MCS guidelines.


The government RHI subsidies will probably (and rightly!) only be awarded to clients who can demonstrate the following:

  • That they have used an MCS accredited installer to put in the GSHP system, and

  • That the installer has made use of the correct design procedures for both the sizing of the heat pump and also the dimensioning of the borehole(s) serving the heat pump.


Both of these areas are covered in the MCS Guidelines referred to above

To be specific, as a minimum, the installer should be able to provide the client with the following two pages, filled out for each installation. These may later be required as proof that the correct design procedures have been followed.

Both of the pages below have been copied from the MIS 3005 (Issue 3) standard issued by MCS.

For larger installations, i.e. > 20 kW, a geological survey and professional borehole array design are strongly recommended

This is what KENSA the manufacturer of British heat pumps says 

The higher the output flow temperature from a heat pump, the more energy the heat pump will use, and the higher the running cost will be. 

Equally, the better insulated a building, the lower the flow temperature required into the heating system to achieve target temperatures.

The test point for heat pumps (EN14511-2) is usually quoted as 0 degrees C coming in from the ground arrays, and 35 degrees C for the flow temperature into an underfloor heating system.

All Kensa heat pumps are shipped with the return water temperature setpoint set at 30 degrees C, which is ideal for underfloor systems mounted in screed. This corresponds to an approximate flow temperature of about 35 degrees C. This is only approximate as the flow rate changes (as zones open or close) and the delta t (or difference between feed and return temperatures from the distribution system) also changes.
A flow temperature of 35 degrees C should be more than sufficient to heat a well-insulated building using underfloor mounted in screed, to its Part L design temperatures. If the building is not well insulated then a higher flow temperature would be required reducing the efficiency of the heat pump. 

If the underfloor is not mounted in screed (i.e. the first floor construction is joisted) then it maybe required to run the heat pump at slighter higher temperatures to drive the heat through the floorboards and any floor coverings. This can be achieved by reprogramming the control following the instructions within the manuals. If the heating distribution system is radiators then the flow temperature again will need to be raised higher. 

However if the return temperature setpoint is raised too high (above 50 degrees C), either because the occupants want a to exceed Part L design temperatures, or the building is not well insulated - then the heat pump will not just run less efficiently, it also means that the heat pump will run for longer. This will extract more heat from the ground arrays than their design intended (cooling them down further). There is also a danger that even if the heat pump can provide the correct power to the system, due to the low flow temperatures, the heating distribution system will not be able to provide the heat into the building. 

The ground is used as a battery. If too much energy is taken in the first half of the winter from the ground or by running the heat pump for too long, then the output will fall later in the winter as the ground arrays get colder and colder. In extreme situations, the ground arrays will get so cold that the antifreeze protection (generally down to -10 degrees C) will be insufficient. Simply adding more antifreeze would not cure the problem, as the viscosity would increase, and the specific heat capacity of the fluid would decrease, so a larger ground circulation pump could also be required.