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Legionnaires’ disease is a serious lung infection (a type of pneumonia) caused by a bacteria called legionella pneumophila and is contracted by breathing in droplets of water (such as from a shower) which contain the Legionella bacteria. Legionnaires' disease can be fatal - especially to people whose immune systems are weak through age or a pre-existing illness.
Although the legionella bacteria can be found in most water systems, the main areas of risk are where the bacteria can multiply and increase to dangerous levels. This is where there is water of between 20 and 45 degrees, where water in the system can become stagnant, and where there is rust, sludge, scale or organic matter for the bacteria to feed upon and multiply.
The majority of modern water systems in residential rental properties fed by condensing combination boilers will not require any action further than the carrying out of a risk assessment. Older gravity-fed water systems with header tanks may require further work to be carried out.
The Health and Safety Executive (HSE) code of practice for legionnaires' disease now includes water systems of a volume of less than 300 litres, bringing residential properties under its auspices and making landlords responsible for risk assessment and any work needed to rectify problems discovered by the assessment.
You (or a suitable third party) need to log all of the component parts of the property's water system and document them. You then examine this log and decide whether there are any places in the system where the legionella bacteria can grow. Examples are little used mixer taps or showers where the water can get stagnant, places where sludge can accumulate, or places where hot water temperature is likely to fall between 20 and 45 degrees centigrade.
Redundant pipes should be bypassed. Hot water cylinders should have their thermostats adjusted to keep water permanently at a hotter temperature than 65 degrees centigrade to kill any legionella bacteria. You should regularly flush showers and taps that aren't used very often with water that is hotter than 65 degrees centigrade. Flush them for at least a couple of minutes. If a property with a gravity fed system is to be left empty for more than a couple of weeks the system should be drained down to avoid stagnation.
You can be prosecuted and heavily fined if your tenant becomes ill and you had not properly risk-assessed the property, so spend half an hour doing it and protect yourself.