Vapour compression cycle

GSHPs transfer energy from outside to inside and useit to provide heating and domestic hot water. In contrast to conventional heating systems, a GSHP system supplies more energy than it draws from primary sources.

A GSHP is at its most efficient when the water is heated to the lowest possible temperature, making it an ideal partner for warm water UFH, facilitating potential flow temperatures of between 35°C and 45°C and enabling the heat pump to reach its best Coefficient of Performance (CoP). This could be as high as 4 to 5, meaning that the heat pump is performing at 400 to 500% efficiency. It is possible for heat pumps to produce water of a higher temperature but this will reduce the CoP.

Integrated solutions

The ground loops of a GSHP system contain a glycol solution. As this is circulated around the buried ground loops it absorbs the heat from the ground and transports it back to the heat pump. The low temperature glycol enters the evaporator where the heat is transferred to the refrigerant. The refrigerant boils, turning into a low-pressure vapour. Next, the vapour passes through a scroll compressor. Once through the compressor the vapour is at a high pressure and the temperature has been raised considerably.

Passing the vapour through a second heat exchanger (the condenser) enables the heat to be extracted at a higher temperature to the primary water for the heating and domestic hot water side of the system. Having removed the heat from the vapour it condenses back to a liquid state, but the high pressure must be released by passing through an expansion valve, allowing the vapour to return to a low pressure liquid state ready for the refrigerant to begin the cycle again.

This whole process is designed to constantly draw the heat from the ground at a steady rate, returning the glycol back through the ground loops at a lower temperature. Therefore it is important that the system is designed to allow the ground to recoup energy removed.

Heat pump characteristics

In simple terms, a GSHP transfers heat from one source to another, cooling down one medium and heating another in the same cycle. Unlike a conventional gas/oil boiler system which is usually oversized to cope with higher load demand on start-up, heat pumps are not intended to be switched on and off.

A heat pump is designed to run on a ‘tick over’ basis to maintain internal temperatures and is available 24 hours a day to bring up the temperature as it drops below the pre-set level. As the output of UFH is carefully matched to the heat loss of each room, it too can run on a ‘tick over’ basis, making it an efficient partner.

Heat pump controls

In order for a GSHP to operate effectively, the choice of heating emitter and the design of the heating system should be undertaken with precision, bearing in mind that the efficiency of the heat pump decreases as the output temperature increases. There are two methods of control; the open flow principle and zoned systems. Both open flow and zoned systems are run with the GSHP in weather compensating mode.

Open flow system

Open flow a system where the UFH zones are not thermostatically regulated. Instead, the system is balanced using the flow valves on the manifold. This ensures that each room receives the heat that it needs.

To achieve this, the heat curve of the heat pump must be looked at, to ensure you are on the right one. The heat curve is chosen as part of the design process, and will indicate what the flow temperature should be based on the outdoor temperature. There are preset curves within heat pumps that can be adjusted. Once the right flow temperature has been achieved, then the UFH flow rates can be balanced.

At commissioning, the flow gauges and adjustment valves on the manifold are set up to attain the design flow rates, but there is a likelihood that user requirements may differ from design parameters, building parameters may vary or some aspect of the installation will mean that the output is slightly different from the expected. At this point the adjustments (or ‘tweaks’) can be made. This can be a slow process. For example, if a particular room is cold, the system can be adjusted, but this means that more flow is being put into that room and slightly less is going into other rooms, so further adjustment must be made and so on and so forth, until the perfect balance is achieved. Overall this is not an ideal process, as neither installer nor customer will want to spend a great deal of time on adjustments.

Zoned systems

To determine the best method of running the heating and domestic hot water system via a ground source heat pump (GSHP) it is essential to understand how the heat pump works

Senior Development Engineer Chris WeightmanAccepting that there is a small difference between design ideals and reality (and although in theory they shouldn’t be needed), the use of thermostats will answer the requirement for control.

Thermostats should provide a fine level of control to give the customer perfect comfort. They may also make the customer more satisfied with the system, as they feel they have more control which should avoid the need for the installer to return to the installation frequently. As the difference between design and reality is likely to be small, efficiency will not be unduly affected.

However, there is potential for some of the thermostats to shut off, and therefore the system will require a buffer tank (allowing the heat pump to continue running at its most efficient). When planning a zoned system the physical size of the buffer tank should also be considered (as it may require a purpose built plant room).