Published on March 9, 2014
A New System Approach to Efficient Central Plant and District Heating design CIBSE AM12:2013 (9.16, p 49) states: “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 .
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.
Danfoss TPV Temperature Chart
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.
CHP and carbon footprints 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.
The 70/40 principle - tried and tested
70/40 CPD Seminars 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.
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