Legionella are bacteria that are water borne and can cause a form of pneumonia which can be fatal.  To minimise legionella in hot water systems (ie the amount of bacteria present), water systems must be in good condition.

Hot water systems are just one of the types of system where Legionella bacteria may live, so hot water systems need to be properly maintained and inspected on a regular basis.  Here are some tips for  control of Legionella in hot water systems.

Legionella Monitoring

The person responsible for maintaining the water system should have had Legionella training so they can check and clean the systems. Particular attention needs to be paid to the sentinel outlets and the temperature at which water is distributed should be monitored. This will prevent Legionella in Hot Water Systems

Stagnant Water

Where water is allowed to stand unused and is warmed up to above 20 degrees, it will start to stagnate. This could create a problem and eventually Legionella in Hot Water Systems because Legionella bacteria will invariably grow.  Stagnation is most likely to occur where services have been removed but the pipework has been left in place, where water services are infrequently used or where too much water is stored.  These conditions should be identified and once identified, should be removed, or action taken to control Legionella growth.


Showers should ideally not be fitted where they are likely to be used less than once a week.  Where they are installed they should be used on a regular basis, or flushed, to help minimise the growth of Legionella bacteria.  Shower heads should be dismantled, cleaned and de-scaled to help maintain them in good condition.

Water Samples

Water samples should be taken and analysed every year, or as defined during the risk assessment.  This is particularly important for high risk water sources, such as stagnant water.  As cases of Legionella result in a full investigation along with prosecutions for any failures in Legionella compliance, it is vital that those responsible for water systems receive Legionella training and understand the risks involved.

Water Temperature

Legionella especially thrives at temperatures between 20 to 45 degrees.  If the temperature reaches below 20 degrees the bacteria lies dormant, and not growing and multiplying.  If the temperature reaches 50 degrees centigrade or higher the bacteria is removed.  Hot water storage systems should supply water at 60 degrees centigrade or more at all times.  If the hot water recirculates around the system, the water should then return at 50 degrees or above at all times.  The resulting hot water should ideally arrive at taps at 50 degrees or more within 1 minute and cold water should arrive at taps at 20 degrees or less within 2 minutes. This criteria will prevent Legionella in Hot Water Systems.

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