Everything you need to know about: ground loops
One part of the fitting of a ground source heat pump that is not often written about is the installation of the ground loops. Nu-Heat looks at some questions received from installers.
How do you size ground loops?
The recently released Microgeneration standard MIS3005 includes a method and data for sizing the ground array for a heat pump from any manufacturer. With data based on relevant weather, varying geology and differing values of heat pump running hours, an installer can work out what output it is possible to expect from the ground (expressed as a W/m² figure).
There are two main requirements to correctly size the ground array:
- The ground has to deliver the peak power load of the system (so that enough energy can be accessed from the ground quickly enough during the coldest conditions). As an example, if the heat pump’s maximum is 8kW, then the ground array must be able to collect 6kW.
- The ground must be capable of providing the annual energy extraction. If too much energy is extracted, the average ground temperature will deteriorate over a number of years to the point where the source cannot supply the required amount of heat which would severely affect the performance of the heat pump system.
The newly published tables are not applicable to systems that are more than 45kW – the limit of what is considered as microgeneration. For larger systems a specialist design company should be consulted.
Is single pipe better than joined lengths?
Another specification of MIS3005 is that the flow of liquid in the pipes must be turbulent, which means that the fluid must swirl and mix. The mixing is important as it means the heat is carried from the edge of the pipe to the middle, transferring more heat into the fluid and making sure that enough is delivered to the heat pump. This is only possibly when the flow is fast enough. However, the higher the speed, the greater the pressure drop, meaning that the heat pump has to work harder, using more energy and possibly requiring a bigger model of heat pump.
The alternative is to split the pipe into lengths that a sensible sized pump can cope with. For example, if a 900m² area of ground has been specified, with pipe at 1m spacing then 900m of pipe is required. This creates a far greater pressure drop at a turbulent flow speed than is practical. A 300m pipe is a sensible solution, so the answer is to install three separate coils to run off one manifold.
Should ground loops be insulated?
In general, ground loops need to be in contact with the soil. However, insulation will be needed where it is important that the ground mustn’t freeze, such as an area close to a house or garden wall. This is because the fluid going through the pipe will drop below 0°C at some point, particularly in the flow and return pipes. In extreme conditions there’s a possibility of the ground freezing around the pipe, which potentially could cause ground ‘heave’ and adversely affect the structure. Where a bad design has been used, it has been known for a garden wall to fall down!
Where there is no available space to place flow and return pipes between the main array and the heat pump far enough apart, one pipe (usually the return pipe) should be insulated so that heat isn’t passed from one to the other.
The best type of insulation is a ‘closed cell’ type.