Can warm water be bad for you?

© Sebastian Kaulitzki |

© Sebastian Kaulitzki |

I was very concerned (but not that surprised) to read about Legionella bacteria being found in the hot water systems of a number of Queensland hospitals and linked to at least one death last month.*

This incident serves as a warning to anyone who has turned down the thermostat on their domestic hot water system to save energy or money or to reduce the risk of scalding – especially if there is anyone at home that is elderly, immunocompromised or a heavy smoker.

The type of Legionella bacteria that can cause Legionnaire’s disease is commonly found in soil and water. It can pose a health risk in warm water systems (especially temperatures between 20 to 45°C) and when it becomes air borne (e.g. in showers and cooling towers).

Even rainwater in above ground tanks and exposed pipes can reach high enough temperatures in warm weather that allow the bacteria to multiply but not high enough to kill them (and bearing in mind that rainwater does not generally contain chlorine).

Solar hot water systems can also be a problem as they may not reach high enough temperatures to kill the bacteria without a gas or other boost.

I will be checking the temperature setting on my system at home and may rethink whether I turn off the hot water when I go away for more than a few days. Fortunately it does not take too long to kill the bacteria at temperatures above 60°C so maybe it’s still okay as long as I give the water enough time to reach the set point before I have a shower…

* e.g.

Categories: Water

Tags: , , , , , ,

2 replies

  1. We are currently undergoing studies into Legionella Bacteria and would like to stay in connection with you on this topic. Our work in the field Rainwater Harvesting with the use of UV disinfection for drinking water has been growing daily and we are ready to advance into cooling tower and boiler disinfection.
    Please reach out to us to connect.
    Thank you.

  2. Hi
    Thanks for your comment. I do not have any particular expertise in this field and rely on others to provide advice.
    Some of the guidance may change as a result of recent incidents, but this 2003 article by Clive Broadbent is one of the best I have seen on Legionella bacteria in warm/hot water systems –
    One of the issues I have discussed with the author is that UV does not provide any residual protection post-treatment. A simple solution is to add chlorine if this is likely to be a problem.
    Electro-chlorination looks like a really promising way to do this in some applications (i.e. adding a salt source – or using salts in the water itself – and passing a current through it to produce chlorine). Some articles about this technology were included in the Handbook of Sustainable Engineering published earlier this year e.g. ‘Chlorine Self-Production Plant Solution for Effluent Water to be Used in Irrigation in Gaza Strip’ ( and ‘Micro Hydro in Emergency Situations: A Sustainable Energy Solution at La Realidad (Chiapas, Mexico)’ (
    Another issue with UV is that it may not be effective if the water is turbid or has poor colour. Although first flush diverters and filters are designed to reduce the inflow of dirt and leaves, these can be sources of bacteria and cause discolouration in rain water.
    Good luck with your studies – cooling towers are a whole other story!!

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