Refer to Legionnaires' disease-Technical guidance-Part 2: The control of legionella bacteria in hot and cold water systems

http://www.hse.gov.uk/pubns/priced/hsg274part2.pdf

2.69 Low storage volume heaters serving hot water outlets should be able to achieve a peak temperature of 50–60 °C and where the thermostat is set at these temperatures for this purpose, staff and other users should be informed not to adjust the heater. A unit which is not capable of achieving this, eg a preset thermostat, should only be used where there is a very high turnover or an alternative control measure is in place.

2.70 Low storage volume heaters, which includes electric showers, often have spray nozzle outlets and these should be inspected, cleaned and descaled as part of the showerhead and hose cleaning regime.

2.71 If these units are not regularly used or set to supply warm water, the risk from legionella is likely to increase dramatically and may increase further, where the units are supplied from a cold water storage tank. The risk assessment should take into account the usage of the units, the susceptibility of those using the units and include a suitable monitoring regime where the risk is considered significant.

Just fit a thermostatic valve so although it arrives at the sentinel at 50 c but discharges at 43 c.

Apologies I see there is no cold water connection. Personally the chances of a Legionella outbreak from a well used sentinel are minute so crank it down to 45c and flush thoroughly through weekly. Alternatively write to your MP and ask why the Water board are not legally required to sterilise Legionella from their supply.

Presently we are supplied with water that is contaminated by Legionella were that not to be the case I suspect the chance of an outbreak would be significantly reduced. Adding further toxic chemicals would be ridiculous however pointing out the failings of Water boards is not.

Legionella is a common (almost universal) microorganism found in most water supplies. Usually is does not pose any risk unless:
1) It is allowed to multiple in warm, still water (approximately 20- 50C), eg in a holding tank or a terminating pipe. Here is grown as part of the biofilm- the natural scum that forms in pipes.
2) And it is made into an aerosol eg in wet cooling system, a shower head, even a fountain.
3) There are vulnerable adults, with weakened immune systems, being exposed to this aerosol.
The ways to control this are:
1. Control the temperature ie keep the water cold or hot
2. Use biocides to kill off some of the bacteria
3. Keep the water flowing.
So what we have here is small water heater with no storage of water and only a limited aerosol. Furthermore the people who might be exposed seem to be relatively healthy. Is there any evidence that there is high load of legionella in the pipework?
If not I’d lower the temperature as others have suggested.
Remember the water supply is provided for a good reason but it must be usable. Water coming out at 50C is not usable for hand washing.
BJC no water supplier in the world is able to deliver totally sterile water through the public supply; it’s just not practicable.

Yup Chris. 50 degrees is not a kill temperature, it's a storage temperature - the time required to hold at that temp in order to kill decreases as you approach and then exceed 60 degrees - but 50 for a few seconds will give legionella little more than a temporary headache.

There is a difference between an instantaneous heater and one with a small storage tank though but regardless, assuming high turnover from very regular use (I'd confirm that evidentially if I were you) then you've likely got it covered. Of course if you can hit 50 degrees then you should, but in this case there is a reasonably practicable argument for not doing so.


As for sterilising the water coming out of your tap........