As you would expect, there are multiple things that effect how efficient or inefficient the district network is. Each issue generally comes down to either increasing or decreasing the network delta T. Small delta T’s equal an inefficient network, large delta T’s equal an efficient network. The greater the delta T the lower the flow rate required to supply a given amount of energy. Typically, the flow temperature will be fixed and therefore, lowering the return temperature should be the focus as this will increase the delta T.
At every stage of the design, installation, commissioning and operation, the focus should be on maximising the delta T. If the result of a design change, system change etc. Would be to reduce the delta T, it should be considered very carefully before the change is made.
It should also be noted that reducing the return temperature will allow the majority of renewable sources to provide a greater part of the total output. Solar and heat pumps, for example, will struggle to increase the temperature much beyond 55°C. If the network return is 55°C or above, they cannot provide any energy into the building. Some ‘higher grade’ heat sources producing higher output temperatures can also suffer. CHP for example will struggle to ‘get rid’ of its heat and the engine may shut down regardless of the thermal or electrical demand of the building.
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