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is it safe to let a water heater to be unplugged for ~3—6 months at a time? (mander.xyz)

submitted 1 month ago* (last edited 1 month ago) by [email protected] to c/[email protected]

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My house has a tankless water for most of the house. Exceptionally, one floor gets hot water from a tank. I rarely need hot water on that floor so I keep the tank unplugged. But when I need a backup shower (e.g. the tankless gets clogged with limescale) I plugin the tank, let it reach a quite high temp, then shower.

Is this risky? I just heard from someone saying they only unpower their water heater for 1 day at a time because of some specific kind of bacteria. I was assuming whatever bacteria colonizes in 6 months or whatever would be killed off when I fire it up. But I know that some bacteria (which goes after spoiling meat) produces toxins, so even when the bacteria is dead there are dangerous chemicals remaining. Is this the same risk with water heaters?

If it’s unsafe, what do I need to do? Do I have to fill the tank with air between uses? Or can I just run the water for as long as needed to get all new water in the tank before powering it?

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[–] [email protected] 8 points 1 month ago (1 children)

Legionella’s disease is probably what the someone was on about. Have a quick scan of the wiki article if you want. Any water that is left in a system and does nothing “may” become an issue. If the system was drained down before hibernation and crucially refilled BEFORE powering up you would be in safer territory immediately. Whoever definitively answers this question for you should have no problem with you asking for their qualification and experience in the water safety field.

[–] [email protected] 2 points 1 month ago (2 children)

Yeah, Wikipedia says 70°C hot water kills them practically instantly: https://en.wikipedia.org/wiki/Legionella#Legionella_control_and_biomonitoring

So, I guess, if you heat it up beyond 70°C and leave it running for a minute before you step under the shower, that would presumably be safe, but I'm not an expert either...

[–] [email protected] 2 points 1 month ago

Pretty sure UK law is/was: hot water stored at 60+, distributed (i.e. out of the tap) at 50+. Cold should be stored and distributed under 20. It’s astonishingly common in the UK for hospitals, hotels, care homes etc to have a tap-running check list where an individual runs unused taps for a set time (30 seconds?) every so often. In my limited experience your shower head is the badboy you need to pay most attention to - but that’s purely anecdotal.

[–] [email protected] 2 points 1 month ago

This is about as definitive as you’ll get online:

https://www.hse.gov.uk/healthservices/legionella.htm

It’s water, after all. Regardless of where you live in the world.