Water Legionella
> comes up, you have no load to offer the solar system.
“Taking the system’s water temperature up to 60 deg C
We may need Shutterstock
is inevitably going to negate some of the solar benefit,
to be careful
but timing is important.”
Martyn Griffiths, technical consultant to the Hot
about any
Water Association, says one solution is the use of
disinfection
smart control systems that look at the temperature
processes
regime at one or two levels in the tank and use fuzzy
logic electronics to ensure that the water is taken up
adopted for
to a sufficient temperature to mitigate the effects of
solar water
legionella.
heating
“This will ensure the right balance between economy
and safety,” he says.
systems.
Coils: designs to keep
However, he argues that it is unclear how big an issue
– Mervyn Smyth
legionella in hot water systems is: “The theoretical risk
bacteria at bay
is there, particularly in existing systems, but nobody A typical solar water heating cylinder has two coils.
can give figures because legionella is often not The lower coil is fed by the solar collector array
diagnosed properly.” and the top coil by the primary heating appliance
David Matthews, chief executive of the Solar Trade – usually a boiler. If the solar heating is unable to
Association, insists there is no evidence that solar water raise the temperature of the water in the cylinder to
heating increases the risk of legionella contamination the temperature needed to feed the heating load,
compared with a conventional boiler, although he the boiler will kick in and take it to the required
concedes that solar can run at a lower temperature. temperature.
“We are conducting due diligence research into However, the top coil can also be a source of
this. With others, we hope to prove that solar does not energy for pasteurisation. Yan Evans, of Baxi, says:
increase the risk of legionella. I imagine we are a year “You can fit a temperature sensor that says: ‘I’m
away from the results of these tests,” he says. not up to temperature, I’m going to call the primary
According to the report Preparatory Study on the Eco- heating appliance [boiler] on to raise the water to
Design of Water Heaters, published in September 60 deg C in the top segment of the cylinder’.
2007 for the European Commission: “Different temperatures mean that the water has
“Despite millions of installed solar different densities, so you can have a stratification
thermal systems in use worldwide, effect within the cylinder. This means the legionella
no cases of legionella have been bacteria could live in different parts of the cylinder.
reported that could be traced Therefore, this sort of system should be fitted with
back to the solar water heater.” a destratification pump to circulate the hot water
Nonetheless, Matthews stresses from the top of the cylinder to the bottom. So there
that it is sensible to take precautions is pasteurisation throughout the cylinder.”
to mitigate any risk. l An alternative system design might involve solar
Solar Heating Design and Installation hot water to preheat a system that incorporates
Guide (Domestic Building Services direct-fired water heaters or a calorifier. In this
Panel: 2007), order code case, the system may not have an immersion
HVSH, is available from heater or second coil because the primary heating
the CIBSE bookshop at appliance is the direct-fired water heater. So there
www.cibse.org is no source of energy within the cylinder to use for
pasteurisation purposes.
Shutterstock
“However,” explains Evans, “calorifiers and
direct-fired hot water storage heaters typically store
water at around 60 deg C. In this case, we would
place a shunt pump between that primary heat
source – the calorifier or water heater – and the pre-
heat cylinder; and a building energy management
system or solar control unit would cause water
between the internal storage of the primary heating
appliance and the pre-heat cylinder to be drawn
off, ensuring pasteurisation. One of the big design
issues here is that the shunt pump needs sufficient
flow rates to take a certain quality of water up to 60
deg C within an hour and hold it there so you have
Different kinds of solar
to make sure the pump is sized correctly.”
thermal systems are available
42 CIBSE Journal August 2009 www.cibsejournal.com
CIBSEaug09 pp40-42 Hygiene.indd 42 23/7/09 15:59:04
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