District heating networks have been identified as having a key role to play in both the UK’s carbon reduction strategy and the European heating and cooling plan. Yet if they are to fulfil their potential they need to be lean on energy consumption. To achieve this, they must operate with low system temperatures – potentially considerably lower than traditional design practice.
Sadly, all too many heating networks have their efficiency compromised by high return water temperatures. This is a symptom of inadequate control, often as a result of poorly balanced radiator circuits, sometimes exacerbated by rushed installation.
Attempts to compensate for these uncontrollable variables often result in high system temperatures, low Delta T, unmanaged pressure variations in the system and compromised performance.
One of the key issues here is that if a radiator is not controlled properly it may become an unintentional bypass. The inevitable result is uneven heat distribution through the system resulting in an overall reduction in efficiency of the entire heat network.
Nor is this situation fully mitigated when heat interface units are used to provide hydraulic separation between the space being heated and the heat network. This is because the return water temperature of the heat network distribution system is still dependent on the return temperatures from the heated spaces.
So where does the responsibility lie for ensuring that heat networks are designed to deliver optimum efficiency? According to CIBSE, it lies with the engineer that designs the system and the building owner/operator.
“Achieving low return temperatures starts with correct selection and balancing of radiators and other heat emitters within the building, which is often the responsibility of the building owner and designer and not the heat network owner/operator.”
– CIBSEs Heat Networks: Code of Practice for the UK (2015)
An engineered solution
One part of the solution is to ensure that the thermostatic radiator valves (TRVs) are both pressure-compensated and temperature-compensated by using an integral differential pressure controller to ensure that only the required flow passes through each radiator in both full and part load conditions. The result is a responsive system where radiator circuits are dynamically balanced regardless of pressure variations or changing demand.
Additionally, as less water circulates through the heating system the performance of the heating circuit pump is optimised, thus adding to the energy savings already being achieved by proper balancing. Noise issues with the heating system are also eliminated.
SAV’s PT40 TRV sets have been designed to minimise installation risk and optimise the performance of heat networks in accordance with CIBSE Heat Networks: Code of Practice for the UK (2015).
They are designed to operate with low flow rates and include integral adjustable apertures that can be pre-set to the required flow rate. Consequently they prevent any single radiator from acting as a system bypass.
The result is an efficient, low temperature heat network that remains at low temperature with:
- Even heat distribution.
- Lower system return temperatures.
- No noise issues.
- Reduced energy waste.