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Every now and then, I have an interruption in the supply of water from the city. In the past, I would simply notice there is no water provided, and that the water meter would stand still.

But I have recently installed myself an electrical water heater (a simple device of 200 L that contains water, and heats it), as a replacement for the previous gas water heater (that had a much lower volume, and was instantaneously heating the water).

Since then, when there is a water interruption, the water is leaving the water tank at a dramatic speed (I see my water meter turning very fast in the WRONG direction).

Should I be worried about this? Anything I should do?

EDIT: Upon further inspection, I was actually mistaken, and the water tank actually stayed full. The whole cold water circuit was emptied, and the water meter kept running because of strong air pressure. Suction from the supply side caused strong air pressure leaving my house's system. I am still confused as to how that could be.

Here's the system I have installed at the cold water entry point of my electrical water heater :

enter image description here

When the problem occurred I turned off the taps of cold and hot water in and out of the water tank. Effectively, this was removing the water tank and the vacuum breaker from the circuit entirely. And yet, the problem kept happening (air was still flowing strongly, going towards the supply). So I think this means it was not a problem with the water heater nor the device on the picture?

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  • Per your edit: that makes no sense, where did the air pressure come from? Plumbing is a sealed system when the various output valves are off.
    – KMJ
    Commented Nov 3, 2022 at 14:42
  • @KMJ The vacuum breaker allows air into the system exactly when there is negative pressure on the supply side (to prevent the tank from being drained).
    – zwol
    Commented Nov 3, 2022 at 15:32
  • @zwol thank you, had not thought of that as an option.
    – KMJ
    Commented Nov 3, 2022 at 15:57
  • @KMJ The asker appears to be in the UK. Storage cisterns are common and often required, so the system would not be sealed.
    – user71659
    Commented Nov 3, 2022 at 17:52
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    Not in the UK myself, but something to be aware of is water quality and contamination. The public water supply is generally kept clean by pressure. Dirt and microbes and other contaminants can't make it in the water supply pipes because of how much pressure the pipes are under. If that pressure is lost, there is a high chance of contamination. I'd avoid using the first bit of water that comes out of the pipe after it comes back.
    – user1558604
    Commented Nov 4, 2022 at 13:16

2 Answers 2

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Three things here.

First: since you self-installed it sounds like you might have missed the vacuum break. Here is some brief discussion of why it is required by the Universal Plumbing Code. Specifically it is called out in 608.7 (in the 2015 code). If the tank can drain itself, something is wrong with your plumbing install. The tank self-draining can result in it burning out or even in rare circumstances imploding.

Second: you might have an additional plumbing problem. Usually it is required to prevent cross-contamination of the water supply by having sufficient backflow prevention. Either your system doesn't have sufficient backflow prevention, or a backflow prevention device has failed. Your plumbing shouldn't be able to drain back in to the water system, regardless of the water pressure coming from the city. Backflow prevention isn't required in every scenario or every place, which is why I say 'might'.

Third: regardless of backflow prevention requirements, this can be dangerous. If the water is flowing back from your house when the city loses water pressure, it could be flowing back from all sorts of other houses as well. That could include water in their irrigation systems that is contaminated with fertilizer or wastewater. You should reach to the municipality and let them know that this is happening. After a loss of water pressure, you might want to avoid consuming water from the system unless you boil it.

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  • Thanks for that. I think I need to install a one way valve.
    – DevShark
    Commented Nov 2, 2022 at 18:34
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    At a minimum add one at the tank per the links if you didn't put one in. It should be cheap and easy to add. Then you might want to consult a plumber about the rest of your system. Good luck!
    – KMJ
    Commented Nov 2, 2022 at 18:44
  • I actually had a one-way valve just before the cold water input of the water tank. It worked properly through the incident. Thank you!
    – DevShark
    Commented Nov 3, 2022 at 12:38
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    @user71659 the requirement is pretty universal AFAIK. Backflow prevention is also called out in chapter 29 of the International Residential Code. It also appears to be covered by several European standards including 806-3, but I'm not familiar enough to reliably point there. Nobody wants the potable water supply contaminated by your sprinkler system draining back.
    – KMJ
    Commented Nov 3, 2022 at 17:57
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    @KMJ The UK has old pipes from the days where water pumps were coal-fired and steam-operated. Often mains pressure there would be considered illegal in the US, you can find systems at 10 PSI. Appliances may be designed for as low as 1.5 PSI. A cistern, basically a giant attic-mounted toilet tank, is needed to provide adequate pressure and flow, and that's where backflow prevention would occur.
    – user71659
    Commented Nov 3, 2022 at 18:06
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If the electric heaters/elements in the tank are not covered by water when on, they will burn out and need replacing.

Should add a backflow preventer(one way valve) in the water line.

The interruption when water flows out probably means the city has an open/broken a line, instead of just turning off a valve that was done before.

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  • Thank you for that. I will try and install a one-way valve. Can the valve be installed about anywhere in the circuit, or is it important it is installed very close to the main water tap from the city?
    – DevShark
    Commented Nov 2, 2022 at 18:34
  • At least between the tank and the main water. Should be close to where the water comes in, but does not need to be too close.
    – crip659
    Commented Nov 2, 2022 at 18:40
  • Just curious - this day and age shouldn't there be a protection against that?
    – Vilx-
    Commented Nov 3, 2022 at 9:59
  • @Vilx- Good chance with a recently built house in the US. OP does give location or age of the house or if the plumbing is in code. If talking about the elements, then most times no protection except for the person to know to turn it off before draining.
    – crip659
    Commented Nov 3, 2022 at 10:17
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    I mean a technical protection inside the device that shuts it down if temperature gets too high. I mean, it's such a simple solution, wouldn't even increase the price much.
    – Vilx-
    Commented Nov 3, 2022 at 10:43

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