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18th May 2017

Testing the water temperature

You’ve been tasked with taking regular tap water temperature as part of your routine legionnaires’ disease presentation strategy. Why do you need to do this and are there any special methods required?

What is Legionella?

Legionellosis is a collective term for diseases caused by legionella bacteria including the most serious Legionnaires’ disease. This is a potentially fatal form of pneumonia and everyone is susceptible to infection. People contract Legionnaires’ disease by inhaling small droplets of water (aerosols), suspended in the air from infected water sources.

Why do I need to take water temperatures?

Controlling the temperature in a water system is an important control measure for preventing the growth of legionella bacteria. They are dormant below 20°c and do not survive above 60°c. The problem is that they thrive and multiply in warm water (i.e. 20°c to 45°c.) Water temperatures are regarded as optimal for legionella control when cold water is kept below 20°c and hot water is kept at 50°c or higher.

Too hot to handle

The risk of Legionnaire’s disease is much reduced by high temperatures. However, you must be mindful of the risk of scalding. These risks are especially high for the old, very young or disabled. Water must be hot enough to prevent the growth of legionella bacteria, but no so hot as to burn users.

A common way to deal with this is to fit thermostatic mixing devices. These take in hot and cold water which they blend to warm water at the tap.

How to measure?

If you have been asked to measure tap water temperatures then its likely there will be a legionella risk assessment in place. This should indicate which outlets should be checked and how often.

The following will help ensure you are doing it the right way:

  • The risk assessment should specify the taps to check and how often. Keep a record of your results in a log book.
  • For cold taps, the temperature is simply measured by placing a temperature probe in the water flow. You should achieve a temperature below 20°c when running the tap for no more than two minutes.
  • For hot taps without thermostatic mixing devices the procedure is the same as the above – except that the water need to be at least 50°c within one minute of running the tap.
  • For thermostatic mixing devices the best way is to measure temperature of the metal pipe running into the mixer whilst running the hot tap for up to one minute – unfortunately however, not all system are set up to make this possible!
  • Make sure that thermometers are always calibrated – and ideally when purchasing devices, look for those with a calibration certificate.

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