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…