Heat pumps involve a different approach to system design compared with other heating systems, says Stephen Andrews.
As they don't rely on a combustible fuel, it is important to plan a long-term strategy for heat pumps over the year. This includes studying the building's requirements for heating and hot water at certain times and working out what the base load requirements will be.

Heat pumps, like all heating systems, work best where there is good insulation, where there are no draughts within a building and where the doors are not left open for long periods.

The first stage is for a SAP calculation to be carried out, which determines the sizing requirements of the heat pump and how effectively this will meet the building's space heating requirements.

The key to designing a heat pump system is to keep it simple. We find that most problems arise because of an over-complicated layout.

To gain a full appreciation of the efficiency of a system, it is important to understand how they work. A heat pump circulates a refrigerant fluid around a circuit containing four elements: evaporator heat exchanger, compressor, condenser heat exchanger and expansion valve.

Heat energy absorbed from the ground or air is transferred to the liquid refrigerant which evaporates to form a gas. This gas is then compressed which causes its temperature to rise. The hot gas passes into the condenser, where it starts to change back to a liquid and as it does so, heat is released and transferred into the building. After passing through the expansion valve, the liquid refrigerant returns to the evaporator and the cycle begins again.

As with all good heating arrangements, we recommend on larger systems that, wherever necessary, heat pumps are cascaded, so that there is no over-reliance on any single unit.

With a master and slave arrangement, heat pumps can be sized to meet the requirements of the project. This means that in the unlikely event of the master stopping working, the other heat pumps are unaffected and the system can continue to operate normally, with another heat pump reconfigured as the master. However, these types of problems are rare if the installation procedure is followed correctly.
Experienced heating engineers should find heat pumps fairly routine to install, service and maintain. While the requirement for annual maintenance should be minimal, we recommend routine servicing, similar to a traditional boiler.

Some clients require automatic monitoring of the performance and again an experienced engineer can include this in the system design. It is possible to access data relating to the heat pump efficiency, in order to analyse what performance it is achieving and this is also a good opportunity for the engineer to spot any preventative maintenance work that may be required before it becomes an issue.

Good practice within a maintenance schedule is to check the refrigerant level of a heat pump at least once a year.