I live in an apartment in the UK. There's an electric "hot water cylinder" (storage tank) to heat water for tap usage. (Not for heating, there are electric radiators for that.)

This hot water cylinder has a thermostat where you can set a temperature from 40°C to 70°C, and whoever installed it set it to 70°C. 70°C seems wasteful to me, 40°C would be enough for my use. Presumably the higher the temperature and the more electricity it uses?

However the manual warns that the thermostat should not be set lower than 60°C to "prevent bacteria growth". Is this a valid concern in practice, or is it more like some kind of abstract health & safety disclaimer? I mean, I'm not going to drink that hot water, I just use it to wash stuff.

1 Answer 1


Yes, higher thermostat temperatures mean more electricity consumption. Also your thermostat manual is right, bacteria growth is a valid concern.

The ideal conditions for the Legionella bacteria to grow is stagnant water between 25 and 45°C. If you breathe in water vapor that contains Legionella, you can develop an infection known as Legionnaires' disease, which is a serious illness. Its fatality rate is around 10% for healthy people and higher for people with other health problems.

This is the reason that drinking water companies often advise customers to let their taps run, when people return from (summer) holidays. With stagnant water and in-house temperatures of 20+°C there is a risk of Legionella build-up in your water pipes.

The easiest way to kill the bacteria in your case is to raise the temperature to 60°C or higher, so what you could do is set the temperature to 40°C by default, but every day set it to the maximum temperature for a few minutes. If you think you'll forget to do that then I would advice you to set it to 60°C.

  • Thanks. Setting the thermostat involves unscrewing a panel, so not really something that can easily be done every day unfortunately.
    – Mirko N.
    Commented Aug 4, 2022 at 10:32

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