low carbon system design “70ºc flow / 40ºc return” - a whole system approach

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) heatsource .

Controlling primary return temperatures from domestic hot water production

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

ChallengeAchieving stable DHW flow temperatures and consistently low primary return temperatures under varying load conditions.

Danfoss TPV Temperature Chart

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.

Danfoss TPV Primed 8ºC Temperature Set-back

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

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.

70/40 CPD Seminars

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