Water Legionella
such as sludge, scale, rust, algae and other organic
matter. Dr Mervyn Smyth of Ulster University, UK,
Shutterstock
sees L8 as a comprehensive document in its detailing
of the control of the legionella bacteria in hot water
systems. “However,” he says, “we may need to be
careful about any disinfection processes adopted for
solar water heating systems in the UK as they may
compromise the collector performance, thus rendering
these systems less economically attractive.
“I guess there is a fine line between optimising
solar water heating systems whilst still ensuring
safe operation – perhaps there is a case for flexibility
on legionella control in solar systems based on the
perceived risk.”
L8 warns that conditions suitable for multiplication
of the organisms include a water temperature of 20 to
45 deg C. To pasteurise – and therefore disinfect – the
cylinder, the water temperature needs to be taken above
45 deg C regularly.
However, says Evans, this heating process can Solar thermal and other renewables have raised concern about bacteria collecting in the systems
be problematic: “The issue with solar thermal and
any other renewable source that you want to run at It is, he says, these “primary” gas-fired boilers or
a low temperature to maximise its performance is water heaters that tend to be used in the anti-legionella
the risk of legionella because of the cylinder’s water cycle in winter months.
temperature.
“With an air-source heat pump, for example, to get Frequency
the maximum coefficient of performance, the outlet Document L8 is not entirely clear about how frequently
temperature needs to be around 35 deg C. However, the anti-legionella cycle should be operated, although
this means the water temperature within parts of the Evans suggests at least once a week for at least an hour
cylinder it is operating in conjunction with is liable to ensure disinfection.
to fall below 45 deg C and within the band at which The frequency of the cycle and the time of day at
legionella can breed.” which pasteurisation occurs have a significant impact
One part of the cylinder is particularly prone to this,
Legionella,
on the performance and operation of the solar thermal
warns Evans: “When you heat water in an indirect
particularly in
system.
cylinder that has a coil inside it, the heated water rises
relation to solar
Says Evans: “Optimum sizing of a solar system
up through the cylinder. But there is always a portion can maximise the solar fraction [the percentage
that is below the coil at the bottom of the cylinder which
thermal water
contribution of the solar system to hot water production
is at a lower temperature. This is also an area in which
heating, comes
over the course of a year] to something between 35 and
sediment resides. This combination creates a potential
up on every
40 per cent. To achieve this level of solar fraction, the
breeding ground for legionella.” solar thermal system should be designed for the daily
project that we
hot water demand.
Length of time
work on.
“However, if you were to pasteurise the water by
Legionella bacteria are killed in a matter of seconds at
– Yan Evans
taking the cylinder water temperature up to 60 deg C
70 deg C, and more than 90 per cent of the bacteria every day, then, effectively, you are not offering any load
are killed after two minutes at 60 deg C. That is why it to the solar system and you are therefore completely
is generally recommended that an anti-legionella cycle negating its economic and environmental benefits.”
should raise the water temperature to at least 60 deg So the frequency of the anti-legionella cycle is a
C for at least an hour to be certain of destroying the critical performance issue. To manage the risk of
legionella bacteria. legionella while also maximising the performance of
However, according to Bob Towse, head of health the solar system, Evans recommends raising the water
and safety at the Heating and Ventilating Contractors temperature once a week to remove the threat from
Association, this can be problematic in solar water the bacteria.
heating systems since, at certain times of the year, the
solar collector array that feeds the indirect cylinder coil Cycle timing
might not heat the water up to 60 deg C. The timing of the anti-legionella cycle is also critical
Towse says: “In the UK during the winter, for to the performance of the system. Evans says: “On the
example, there is only sufficient irradiation for solar face of it, it may seem a good time to run the cycle at
thermal systems to satisfy around 20 per cent of the midnight in an office block, for example, when there
hot-water demand. The remainder is usually provided is nobody in the building. However, there is no draw-
by gas-fired boilers or water heaters.” off during the night, so the next day, when the sun >
www.cibsejournal.com August 2009 CIBSE Journal 41
CIBSEaug09 pp40-42 Hygiene.indd 41 23/7/09 15:58:46
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