G : Groundworks


This term describes the joining together of the boreholes to provide circulatory system for the thermal fluid, including a main feed to, and from, the heat pump.

Not the most exciting topic, but a vital part of any geothermal borehole installation. Also, a significant cost element, which can add 20% or more to the cost of the boreholes themselves (including the cost of the manifold)

Correct installation of the headerworks is important for any borehole installation, but particularly so for multiple boreholes where the flow balancing between boreholes is essential to reaching the design efficiency of the borehole array.

Synergy has wide experience at providing headerworks for both large commercial and small domestic projects.

Obviously, with one borehole, i.e. for a small heat pump, i.e. 5-7 kW, this is the simple matter of joining the borehole directly to the heat pump. However, the basic principles we adhere to still apply.

The pipework between the borehole and the heat pump will be installed by us as follows.

1) The pipework will be laid in trenches approximately 1.2 metres deep. If there is any risk of sharp stones, flints etc damaging the PE100 pipes, these will be laid on a protective sand bad approximately 2-3 inches deep. The pipes will then be covered by a sand layer of similar depth. Out and return pipes will be laid as far apart as possible, i.e. on opposite sides of the trench.

2) All joints between the borehole and the heat pump (or manifold) will be made using electrofusion welds. Compression fittings are more susceptible to leakage over the long term. You should bear in mind that these joints will be buried underground, often beneath drives, paving etc. and therefore hidden. Future leakages will be hard to trace and very costly to repair.

3) Careful consideration should be given to the need to insulate the pipe runs. This is particularly relevant if lack of space dictates that the out and inflow pipes must be less than 18 inches apart. In this case it is advisable that at least one of the pipe runs be insulated (usually with Armaflex) We will give you a price for this option.

Multiple Boreholes

All of the above points apply. In addition there is a need to balance and equalise the flows through the individual borehole loops, so that the heat extraction is shared equally amongst the boreholes.

This is particularly important when the boreholes are at widely differing distances from the manifold which is the convergance point for the various flows.

This is achieved by a balancing manifold which is mounted between the boreholes and the heat pump. Individual site circumstances will determine whether the manifold is located underground in a chamber or within the building to be heated.

The manifold must possess regulating valves and flow meters for each borehole to enable the installer to equalise the flow of the heat transfer fluid between the different boreholes. If this is not done, there is a severe risk that one or more boreholes will be over utilised, leading to ground freezing and loss of system efficiency.

Should the balancing manifold be located underground then the choice is between location in a concrete/building block chamber, or, the use of a factory prefabricated manifold chamber which incorporates the manifold. (the solution that we normally prefer, the advantages of this system are detailed below)

Manifold, Manifold Chamber Choice

The manifolds and manifold chambers that we use are fitted with all the necessary valves and flow meters to allow easy adjustment of the complete system. Wherever possible we use prefabricated chambers incorporating manifolds. These units are a little more expensive to buy but have distinct advantages over the more traditional manifolds which are assembled and fitted on site.

  • The main advantages are: Prefabricated chambers are more watertight and do not require the construction of concrete or breeze block chambers on site, thus reducing labour cost and time for installation.
  • Prefabricated manifolds are assembled and tested under factory conditions rather than being assembled under building site conditions. This ensures added reliability.

Header Pipe Installation

Header Pipes are the pipes running between the manifold and the heat pump. These pipes are in most cases 63mm in diameter but can be 90mm dia for larger installations. For single boreholes the 40 or 32mm borehole probe diameter will suffice.

Header pipes can be brought into the building through holes cut in the walls. To seal these holes and prevent future ingress of water Synergy can use hi tech adjustable "Ringraumdichtungen" (Annular seals), imported from Germany.

Synergy can, if required, install these pipes and as part of the commissioning described below, fill the pipes with thermal fluid


All the boreholes that we install will be pressure tested and a Loop Test Certificate made available to the client.

If we also install the interconnecting pipework and manifold, these will also be pressure tested.

What should happen next is that the complete pipework system be cleaned, sanitised, flushed and refilled with thermal fluid (Glycol solution) containing a long term biocide or bacterial growth inhibitor.

Sometimes this process is neglected and corners are cut, biocides not added etc. This can significantly endanger the long term function of the heat pump system. Bacterial growth within the pipework/boreholes can often degrade the ability of the pump to pump the fluid around the boreholes and through the heat exchanger. Bacterial slime deposits will also degrade the function of the heat exchanger itself.

There are salutary examples of large systems grinding to a costly halt after only a relatively short period of operation.

Commissioning the collector system including the manifold can also be performed by Synergy. This will entail the flushing of the complete system with a biocide solution to prevent future fungal growth which can impair or even destroy future system function.

After flushing we would then fill the system will a high quality proprietary thermal fluid mixed with bacterial growth inhibitor or biocide.

Both of these processes are carried out according to a rigorously defined procedure.

We would usually fit the header pipes inside the building with stopcock valves so that the the heat pump installer can easily connect the heat pump unit to the filled and prepared system at a later stage. (see below)

This flushing/filling process takes time, specialist high throughput pumping equipment and significant quantities of biocides and glycol based fluids which are not inexpensive. If Synergy are asked to do this, it will typically incur a cost of around 10-15% of the borehole drilling cost.



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