System design temperatures “70°C flow / 40°C return”* A whole system approach to Low Carbon Central Plant

93469 70/40 Newsletter.indd

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CIBSE AM12/2013:

“It is preferable to adopt a variable volume control system for the DH system and all of the building heating systems connected. This will ensure that pumping energy is minimised through reducing the volume of water to be pumped and the pressure drops to be met, and also reduces heat losses through ensuring that return temperatures remain low under part load conditions.”

“It is recommended that, for new systems, radiator circuit temperatures of 70ºC (flow) and 40°C(return) are used with a maximum return temperature of 25°C from instantaneous domestic hot water heat exchangers.”

* 70/40 is well suited to new radiator systems at maximum load conditions. In part load conditions, flow temperatures should be reduced using weather compensation. The main characteristic of a “70/40” (60/30 or other) system is a large ΔT and low return temperature to best suit the lead (low carbon) heat source.

Central Plant
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Controlling primary return temperatures from domestic hot water production

Objective: Maximum return temperatures of 25°C from DHW instantaneous heat exchangers. (CIBSE AM12:2013 & GLA District Heating Manual for London, 2013)

Challenge: Achieving stable DHW flow temperatures and consistently low primary return temperatures under varying load conditions.

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Danfoss TPV Primed 8ºC Temperature Set-back

The Primed Temperature Set-back lowers the temperature at which the plate heat exchanger is kept warm with app. 8 °C helping to reduce primary return temperatures and system distribution losses.

Controlling primary return temperatures from heating circuits

Objective: It is recommended that, for new systems, radiator circuit temperatures of 70ºC flow and 40ºC return are used. (CIBSE AM12:2013)

Challenge: Protecting space heating control valves from fluctuating primary system pressures.

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Tried and tested

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