Ground Source Heat Pumps
Ground Source Heat Pumps collects thermal energy from the ground to produce hot water for heating and domestic hot water. The ground collector is in the form of horizontal loops, pipework buried in the ground at a depth of 1.2 meters or vertical boreholes with a depth of 80-150 meters. A system mainly consists of the heat pump unit, a hot water cylinder (connected to the hot water taps), a buffer tank (connected to the heating distribution), expansion vessels and the ground collector. The ground collector consists of the ground loops (horizontal or boreholes) a ground manifold and connecting pipework. The heat pump is located internally, in a weather protected space.
As with all types of heat pumps, it is important to specify the right size (kW) heat pump. For this, a room by room heat loss assessment of the property is required. A heat pump produces less hot water than a boiler and the lower temperature it can operate at, the more efficient the system is. Therefore, the overall efficiency of the heating system requires heating distribution (underfloor heating, radiators or fan coil units) which can deliver comfortable room temperatures at low operating temperatures.
We are often asked about the difference between ground source and air source heat pumps. A ground source system is considerably more expensive than an air source one, but ground source is usually a better investment on the long term for the following reasons:
In order determine the right size of the heat pump (in kW) and in the case of ground source, the right size ground collector, we do a room by room heat loss calculation. For new built properties, this is also required for the design of the underfloor heating system and/or radiators.
The physics of heat pumps is the same as that of a fridge or a freezer. In all refrigeration processes, heat is produced as a bi-product. In a heat pump system, the ground (in case of ground source) or outside air (in the case of air source) and the heat produced is used for space heating and hot water in a property.
In most cases, the heat pump is sized to cover the whole heating load of the property and then no back up is required. This is called a monovalent system. A heat pump can also work in tandem with a boiler. This is called a bivalent system. In this case, the boiler is controlled by the heat pump in such a way that the heat pump is the master and the boiler is the slave.
Boreholes are always more the more expensive option. Horizontal loops require quite a lot more space. For a new build property, around twice the total floor area and for older properties up to five times the total floor area. In a garden with trees, the available space for collector pipes is reduced as roots have to be avoided. If a paddock or a filed is available, it is usually the best option. The efficiency of a borehole system is the same as that of a horizontal loop system, provided that the size of the collector is correct. If passive cooling is required, boreholes are a better option, as the temperature in the borehole is more constant over the year.
Horizontal loops can be either in the form of slinkies; 1 meter wide coils, typically consisting of 350 m of 32 mm pipes stretched over 50 m or straight loops (laid in a similar pattern as underfloor pipes). Sized correctly, both systems can provide enough thermal energy to the heat pump. Straight loops extract heat in a more uniform way, and it is therefore usually Better Planet’s recommendation. Sometimes slinkies can be the best option when trenching is complicated.
We always recommend to lay the pipes at the depth of 1.2 meters. For straight loops, the minimum pipe separation is 1 meter and for slinkies 5 meters.
For our projects, boreholes are drilled between 80 and 150 meters. The depth is related to the local geology and the type of drilling rig. For heat pump sized over 6kW, several boreholes are required and they need to be separated by at least 6 meters.
The installation of a ground source heat pump or a water source heat pump on domestic premises is usually considered to be permitted development, not needing an application for planning permission. If you live in a listed building or a conservation area you should contact your council to check on local requirements.