Installation and testing
Installation of the heat pump system, and especially the ground heat exchanger, needs to be carefully programmed so that it does not interfere with – or delay – any other construction activities. The time taken for installation depends on the soil conditions, length of pipe, equipment required and weather conditions. Typically, installation of a vertical or horizontal ground coil for domestic applications can be completed in one to two days. Prior to any excavation it is important to locate and protect any buried utilities, drainage pipes etc.
The GSHP manufacturer’s procedures must be followed. The installation of horizontal heat exchangers is relatively straightforward, but heat exchangers require highly specialist knowledge – not just by the drilling contractor, but also regarding pipe specification, joints, grouting etc. The ground heat exchanger should be installed by professionals who have preferably undergone training by manufacturers, or other recognised authorities such as the International GSHP Association.
When installing the ground heat exchanger it is important to ensure good long-term thermal contact with the ground. Horizontal loops are usually laid on a bed of sand and then covered with a further 150mm layer of sand for protection. Care must be taken to avoid damage when backfilling, and the backfill material should be screened for rocks, stones etc.
For vertical heat exchangers, the space between the borehole wall and the inserted pipes is backfilled with a suitable grout material that is pumped from the bottom of the borehole. Low hydraulic permeability, high thermal conductivity grout (e.g. ‘high solids’ Bentonite), or a thermally enhanced, low permeability grout is used. Grouting over the full borehole length is required unless dispensation has been obtained from the Environment Agency. This not only provides good thermal contact but also prevents any vertical migration of groundwater.
It is recommended that the ground heat exchanger is made from a continuous loop of pipe. Any subsurface connections in high density polyethylene pipe should be made using heat fusion techniques in accordance with relevant standards. For DX systems, work involving the refrigerant can only be carried out by personnel and companies certified to do this.
External pipework must be insulated within 1.5m of any wall, structure or water pipes, and sleeved where it enters the house. When the heat pump is delivering heat, the ground loop circuit will normally be operating below the building interior’s dew point temperature.
Good quality insulation and vapour sealing of internal pipework and fittings in this circuit is therefore essential to minimise the risks, and the pipework should be configured so as to avoid potential damage if any condensation still occurs. A strainer, preferably a removable one, should be fitted. Also, it is good practice to fit a pressure gauge and pressure relief valve. Any arrangement for topping up the ground loop must not be permanently connected to the mains cold water supply. Warning tape should be installed over all buried pipes.
The ground loop should be pressure tested before installation in the ground (this may be done prior to delivery) and again after installation. The loop should be flushed and purged of all air before being charged with antifreeze, and pressurised ready for connection to the heat pump. The antifreeze must be pre-mixed with water before it is added – unless there is a single loop, or the configuration of multiple loops allows individual loops or boreholes to be valved off and filled separately.