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Requirement Calculations

These must be conducted by an experienced professional and not a high pressure salesman. If not done correctly, the result can all too often be a heat pump sale with an underestimation of the borehole requirement, leading to a negation of the reason for going for the technology in the first place i.e. lower energy costs.

The following, very approximate calculation is for a normal residential dwelling with no special features, e.g. heated swimming pool, high ceilinged rooms etc.

This calculation aid is intended as a rough guide only and is no substitute for an assessment by a qualified heating engineer and/or a SAP assessment and report

It is assumed, rather simplistically, that the quality of house insulation is dictated by the age of the house, i.e. when it was built. If your house has had an upgrade of insulation quality then you can adjust this assumed figure accordingly

STEP 1

Please assign a figure to your house according to its age as shown in the table below

Please note that the most energy efficient house building technology of today "Eco House" can reduce this figure to 30 or even less.

STEP 2

Next, please add up the total floor area (In square metres please) of the house to be heated (If you do not wish a floor e.g. upper floor, to be heated by the Ground Source Heat Pump, then do not include this area

Let us say that this figure comes to 260 square metres in total. i.e. Lounge 40, +Study 10, +dining room 14 +etc etc.

STEP 3

If your house was built in 2001, then the calculation is 

80 * 260 = 20800, this is the very approximate number of watts required to heat your house. This equates to 

20.8 kiloWatts or

20.8 kW

This is the approximate size of heat pump that you should look for.

Alternatively, if a SAP Report is available for your house

STEP 1

Please look at the line and figure designated (39)

Immediately above this you will see a row of twelve values, one for each month. Look at the first value (for January, the coldest month).

This value could look like 443.89

STEP 2

Multiply this value by 24

This is the heating capacity of the heat pump required in watts, e.g. a figure such as 10653.36

This means that you need a heat pump of approximately

 10.6 kW

to heat your house

BOREHOLE DIMENSIONING

How many boreholes will I need for this size of heat pump?

Please remember the value that you have arrived at above

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. dry loose gravel) and 60-70 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 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 35 w/m. The larger your installation, i.e. the more boreholes you require, the more important it is to get a correct value resulting from a geological survey or TRT (Thermal Response Test)

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 35W/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 1 kW input, 4 kW of heat are output. Or to look at it another way, 1 kW of electricity put in (to drive the pump etc.) liberates 4 from the ground, a net gain of 3, hence the following calculation.
Energy to be provided by ground source = 10 / 4 * 3 = 7.5 kW (7500W)
Energy yield per metre of active collector 35 W

Why 35w/m?

This assumption depends upon geology, below is a summarized extract from the latest publication


Collector length required is therefore 7500/35 = 215 m

Please note, this can vary between a minimum of 125m and a maximum of 300m depending on the ground characteristics as referred to above

(This will be two boreholes of 107.5 metres)

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 7m apart to avoid thermal linkage (and efficiency loss) between holes.

This value can also vary according to the ground characteristics. 7m is a good conservative average

The table below gives an indication only of requirement based upon GSHP capacity

 

 

 

 

 

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.