Our house has a gas boiler that's connected to both a closed-loop central heating system, and to a system involving a hot water tank and a cistern (through a valve that determines whether it indirectly heats the hot water in the tank, or the central heating loop).

The valve in question was broken for a long time, meaning that only the central heating system was working, and not the hot water (any attempt to turn on the water heater would instead send heat into the central heating due to the malfunctioning valve). We didn't turn the hot taps on at all during this time, apart from occasionally (but too infrequently) turning them on to flush the system.

We've since replaced the valve, but as a consequence, the hot water system has been unused for several years (we've been letting unheated water flow through it twice a year or so in the hope of preventing anything building up there, but that probably isn't enough). So I'm worried that the hot water system might be unsafe to use due to, e.g., bacteria or mold building up within the hot water system, or for some other reason that causes hot water systems that have been unused for years to become dangerous.

The information I've been able to find on this subject implies that there's some sort of need to prevent hot water systems becoming stagnant, and to do some sort of preparation before starting to use them after periods of unuse (when doing so is unavoidable), but I haven't been able to find out about the details (and most of the advice online is directed at much larger buildings).

What steps should I take before starting to use the hot water system again, in order to ensure that the resulting water is safe?

2 Answers 2


Unless you have been deliberately avoiding the hot on your taps/valves while you had only cold water, you have probably run water though more than you think - but perhaps you have. Might depend on your valve types. I know I usually end up in the middle of my one-lever valves when the water heater is off, and at that point the water heater looks like a wide spot in the pipe.

In any case, there's not likely to be a lot to worry about if your water source is clean - legionella is more of a problem with warm than cold (or hot enough to kill it) water, for instance.

If it makes you feel better to flush with bleach or peroxide to "kill everything" then go ahead and do that. To my view, it's no different than the inside of your cold water piping at this point, and of no more concern, so I'd turn on the heat and use it - but you'll find (a few) people here and elsewhere that are sure the inside of your cold water piping is a scary place requiring regular killing, too.

"Stagnant" water is more of a thing when you are considering a pool (exposed to whatever lands in it) with no flow, .vs. a stream of "running" water, than when water is in a closed water distribution and storage system.

  • We were deliberately avoiding the hot taps, because the system had been turned off so long (and because they wouldn't produce hot water anyway). I've edited that information into the question.
    – ais523
    Nov 30, 2021 at 16:27
  • I have to totally agree with Ecnerwal. Unless you have odors coming from the hot water taps, you're probably fine. + Nov 30, 2021 at 16:41
  • 1
    If you're concerned about water health, you could contact your local health department and see about taking in a water sample and having them test it for contaminants or other unsafe water conditions.
    – Milwrdfan
    Nov 30, 2021 at 21:32
  • Odds are good that just flushing all the hot water lines by running water through each hot water tap until you're getting hot water will be sufficient. If it makes you feel better, walk through the house, opening every hot tap, then go back around and close them all in the same order. By the time you're done, you'll have run multiple gallons through each tap and should be good to go.
    – FreeMan
    Dec 1, 2021 at 16:44

A warm water net which is not used for months can become a breeding ground for a zoo of dangerous bacterias (especially for older people, people with reduced immune response etc.), among which legionellas are not the only kind.

Even after heating up (minimum 60°C for 10 minutes) and flushing the complete warm water net, the cold water lines could be still home for dangerous bacterias, since they found their way back from the warm water lines up to the cold water lines.

Other stagnant or T-lines (to breathers, expansion tanks for drinking water, infrequently used garden outlets etc.) are the place where bacterias can settle in big numbers.

Mandatory back flow preventers between the transition from the cold to the warm net (mostly installed before the boiler) help, but are no guarantee for uncontaminated cold water lines.

Indication may be sore eyes after showering, which is not the case at other showers in other buildings.

In that case, also the cold water net should be decontaminated, which is done by acids that are matched to the tube and boiler material. Sealings and other elements in cold water nets are not always made for higher temperatures, thus preventing sterilization by heat.

Here are some information to lower the risk of contamination, but for the European jurisdiction and conditions.

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