“Delta T Design”
In order to realise the predicted energy saving benefits of commercial heating systems and heating networks, it is essential that pipework distribution systems are designed with proper regard to system operating temperatures.
“Delta T” is an acronym for the difference in temperature between the flow and return water in a piped heating distribution system. The selection and maintenance of system flow and return temperatures should be the over-riding design consideration that has to take priority over all others if the energy and cost saving potential of the system are to be realised.
In cases where the anticipated energy savings of heating distribution systems fed from low carbon heat sources fail to materialise, it is almost always choice of delta T, or the inability to maintain the delta T that is wrong.
There are three key objectives of successful delta T designs:
- The operating delta T across heat source should be as close as possible to their optimal design values. This will enable them to operate at close to the peak efficiencies.
- The delta T across secondary distribution circuits should be maximised. This will give rise to lower system flow rates, smaller pipes and smaller pumps that consume less energy. Furthermore, buffer vessels or thermal stores can be reduced in size relative to the size of the system.
- Secondary flow and return water temperatures must be maintained at as low values as possible. This will help to minimise pipe distribution heat losses.
Primary circuit design
Proper design of the primary circuit is essential. Most low carbon heat sources have specific flow and return water temperature requirements that enable them to perform at their optimal efficiencies. The aim should be to ensure that these conditions are maintained for the majority of the operating period.
Table 1 shows typical recommended operating temperatures for different types of low carbon heat source.
Table 1: Typical heat source operating temperatures
It is likely that different types of heat source will be used in the same system (such as gas boilers providing back-up for a biomass boiler or CHP unit). Where this is the case then in the spirit of Part L of the Building Regulations, the lead heat source should be the one that has the lowest carbon emissions. It then follows that the primary circuit flow temperature should be dictated by the lead heat source e.g. if water at 80°C is available from a CHP unit, then this should be the design flow temperature for the primary circuit.
SAV have put together a CIBSE-accredited CPD Seminar, which explores the main issues arising from a 70/40 approach to design. The agenda for this is given under the CPD Seminars tab further along this website. The 70/40 method can be expected to have far-reaching implications, and discussions which are a normal part of each Seminar will sharpen your insight into developments in this field.
If you would like to explore the potential offered by the 70/40 method to any building services project, please feel free to provide some information via the Contact Us page, or by phone / e-mail. We would then assess how best to develop the ideas further, either by CPD Seminar, round table discussion, site visit or other communication.
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