M1 MMC Sept '16 cc

Industry catching on to the importance
of reducing return temperatures
The heating and plumbing industry is becoming increasingly aware of the importance of lowering return temperatures
to improve the efficiency and performance of central plant and heat network systems.
For a number of years Altecnic, the UK’s
leading supplier of heat interface units
(HIUs) has been promoting this
approach, lowering the return temperature
will increase the delta-T so long as the
temperature stays the same.
There are two reasons why this is important, it
increases the proportion of energy that can be
supplied by the renewable energy source out
of the total energy demand, and reduces the
flow rate around the primary system. It’s
therefore crucially important and it is
becoming the overriding starting point in
central plant and heat network design,
according to Neil Parry, the company’s
National Specification Manager.
“Renewable heat sources, including heat
pumps and solar thermal, can only really
generate constant temperatures of around
50oC and if the system return is above this
figure then the renewables can’t support the
system,” he explained.
“When we look at Combined Heat and Power
(CHP)-led systems, high return temperatures
restrict the CHP’s ability to cool the engine. As
a result the engine switches off, stopping both
electricity production and heat output, leaving
an expensive, non-functioning box taking up
space in the plant-room.”
However, lowering the primary flow rate by
increasing the delta-T is even more important
as it affects both capital expenditure and
operating expenditure.
“Meeting the building’s energy demand with a
lower flow rate allows the primary pipe-work
to be reduced in diameter,” Neil continued.
“The lower flow rate also reduces buffer vessel
size and pump size. Clearly smaller pumps,
vessels and pipework save on capital spend,
but the operating costs are also dramatically
reduced. As a result it’s not uncommon to see
pipe diameters being reduced by two sizes or
more when the calculations are recalculated
with the correct delta-T in combination with
the installation of an Altecnic SmartSat
electronic HIU which assists in providing a very
wide delta-T.”
to see that something that Altecnic has been
promoting for many years is finally getting the
focus from the industry that it so readily
deserves!”
Top Tips
‘The Golden Rules of efficient heat
networks’
• Select a HIU that has been designed
specifically for low return temperatures.
• Select a HIU with fast acting control valves
suitable for the maximum differential
pressure the system will produce.
• Ensure that the sizing of the system
(pump, pipe-work, buffer vessel) has been
done to take advantage of these low
return temperatures.
• Utilise the correct hot water diversity
factors. Our recommendation is the
Danish Standard DS439 (also preferred by
CIBSE and BSRIA).
• Keep the primary system free of other
flow restrictions and control valves that
may take the valve authority away from
the HIU’s control valve.
• Switch off the HIU’s preheat wherever
possible.
• Minimise the number and correctly
control the system bypasses.
• Ensure that the pump is controlled
correctly, from the index(s) apartment(s).
• Integrate a plant-room thermal store to
cover for the peak demands on the
network.
• Heavily insulate the primary pipework,
at least to the relevant British Standard
preferably more.
Neil added: “As an added bonus, this approach
also reduces the risk of one of the most
common complaints associated with district
heating systems – overheating in corridors
caused by oversized pipes, high pump flow
rates and high return temperatures. It’s great
For more information contact
Neil Parry, National Specification Manager
07800 865567 neil.parry@altecnic.co.uk
www.altecnic.co.uk
30 MMC
Sep/Oct 2016 MI