13 Jul 2018

Development utilises water supply boreholes via Open Loop Ground Source Heat Pumps

‘Green Apple Award’ winning development utilises water supply boreholes to provide drinking water, grey water and heating via Open Loop Ground Source Heat Pumps.

At small luxury development in south Buckinghamshire, open-loop ground source heat pumps use water from the chalk aquifer to provide the renewable heat for the heating and hot water systems. The water is also used for domestic consumption (subject to analyses and suitable filtration) and for irrigation purposes.

The company were approached in 2011 by Acanthus Developments and asked to tender for the installation of three water supply boreholes to serve each of three prestige properties. Closed loop ground source heating was being considered however, when we presented the idea that the availability of a reliable ground water supply from the chalk aquifer could also provide renewable heating and hot water to the properties, the client decided to install an open-loop ground source heat pump at each property. The development went on to win a Green Apple Award in recognition of the Sustainable Nature of the development and the Architectural Design Excellence of the properties.

Using the postcode of the site to assess the hydro-geology, a quotation was submitted for the construction of both the water supply borehole and soakaway at each location. Both the water supply boreholes and soakaways were constructed and water pumped from one to the other to prove the worthiness of both before and payment was requested. It was several months later, once the construction of the properties had been completed, that the heat pumps could be installed and commissioned. In line with the MCS each renewable heat installation is covered by a workmanship warranty. Separate annual maintenance contracts for each of the renewable heating systems were provided.

The Renewable Heating Systems:

Working from building plans and SAP reports we were able to calculate the heat demand of each property and so were able to identify the size of heat pump required at each. The quote allowed for the supply and installation of Soleco three-phase heat pumps; two 18-kW and one 26-kW. Three-phase power supplies are an advantage as it means the borehole pumps and heat pump are cheaper to run.

The water supply and soakaway boreholes were drilled using a cable-tool percussion drilling rig – the method preferred by the Environment Agency when drilling into the chalk aquifer. Cable-Tool percussion drilling is a cleaner way of drilling than the rotary option. While rotary drilling can be much quicker and therefore cheaper, often the backwash of the drilling process means that fissures can become blocked and therefore the flow of water affected. Rotary drilled water supply boreholes often require ‘treatment’ to re-open the fissures to allow the flow-rate to recover. This treatment is generally an additional expense.

As cable-tool percussion ‘cuts’ through the strata cleanly and no backwashing of cuttings is required, the fissures are unaffected by the drilling process and remain open. Very rarely is treatment required immediately after drilling when using cable-tool percussion.

Borehole logs were submitted to both the British Geological Survey and Environment Agency to protect the abstractions from derogation by a third party.

An advantage of open-loop ground source heat pump installations is that the discharge water can feed a harvesting tank, from where it can be pumped for garden irrigation or other ‘grey-water’ uses. Harvesting tanks that are only connected to a rainwater system will quickly run dry during a prolonged dry spell as they are not replenished, whereas a harvesting tank fed from a heat pump will replenish whenever the heat pump is in operation.

Work commenced in early 2012 and the boreholes were constructed just after the original buildings had been demolished. The borehole pumps were left in-situ to provide build-water until the houses had been erected, after which the heat pumps were installed and the systems commissioned.

From the experience gained on the contract, plumbing and electrical guides were designed by our qualified plumber/electrician/heating engineer. These guides are handed to the client for distribution to their chosen plumbing and electrical contractors. The guides have been written by a fully qualified professional in conjunction with the heat pump manufacturer and inform the heating distribution system installers of both the plumbing requirements and the electrical requirements of the heat pump. This ensures that while work is continuing on the build (and we are away from site), those on site have the information to hand regarding system requirements, providing confidence that all required connections are in the correct location making the installation and commissioning of the heat pump a straightforward process.

The Potable Water Supply

The water supply boreholes also provide a ‘potable supply’ for use in the home. To ensure that the water was suitable for consumption, the borehole water first had to be tested to identify what type of treatment would be required. H.D Services sent a sample for analysis and upon receiving the results appropriate treatment was recommended and supplied. The water for the potable supply is diverted prior to entering the heat pump and is passed through an Ultra Violet filter prior to distribution around the house. At the kitchen tap, a point of use Reverse Osmosis system was fitted to provide drinking quality water. There is a maintenance agreement in place to ensure a continued supply of suitable drinking water.

Work was completed in late 2012 and all three of the heat pump installations were commissioned successfully. Although the whole job, from quotation to commissioning, took over a year we worked closely with the client to ensure that the process was as straightforward as possible.

enquiries@thehdgroup.co.uk     www.hdservicesltd.co.uk      01494 792000

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