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I'm a new homeowner so maybe my understanding is off. My question is, why would turning off the cold water intake valve on the top of the heater turn off cold water to the entire house after about 12 hours?

I thought turning off the cold water intake would gradually shut off the hot water, not cold.

Context: We have an ancient (from 1989!) electric water heater in the basement of our townhouse that popped a leak and is going to be replaced, hopefully within a few days, hence me turning it off.

Here's the cold water valve

And here's the entire heater, the valve is at the top in shadow

I turned the power off to our water heater and turned off the cold water intake last night; when I got up this morning, the sinks and the toilets were working fine (except for the hot water obviously) but a few hours later normal water flow to the cold water faucets and the toilets also stopped working (so no flushing!). When we turned the cold water valve to the water heater back on cold water was immediately restored.

  • HI and welcome. Are you on a well? Sounds like you turned off the well pump's power in addition to the HWH. – mike65535 Jan 14 at 16:20
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    Thank you for the greeting and nope; we live in a townhouse that gets city water. The closest thing to a well we have is a sump pump haha. – VivaLebowski Jan 14 at 16:27
  • Simple question: does turning the water back on to the heater restore water to the house? – Machavity Jan 14 at 16:31
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    If the cold water supply to the water heater was turned off, then you'd lose hot water immediately. – mike65535 Jan 14 at 16:32
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    I think the word you're looking for is "pedantic". :D :D Apparently you did turn off the correct valve and this is quite a mystery. – isherwood Jan 14 at 17:23
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If that is truly the case as you reported it, something is seriously wrong with the design of your plumbing system and you are mixing hot and cold water all of the time, which would be costing you a fortune in electricity bills.

Are you sure it was ALL of the cold water lines that were off? On things like showers, bath faucets and in some places, sinks, there is an anti-scalding / temperature balance valve used to mix cold and hot automatically. With no hot water at all, it may have interpreted that as there being too much cold water, so it kept reducing the cold water in an attempt to balance the temperature. But that would typically not be associated with your toilets. MAYBE, if you are in an extreme environment where the toilets might freeze, someone did that to put a little warm water into the toilets too, but I have never seen that.

  • Just thought of something: if the hot water pipes were run right next to the cold water pipes in an outside wall, the loss of hot water may have allowed the cold water pipes to freeze! That would explain why it took a day to take effect. – J. Raefield Jan 14 at 22:19
  • Except when we turned the cold water valve to the heater back on, cold water flow was restored immediately. Our townhouse has two houses on either side and we haven't experienced any freezing issues yet, so I don't think that's an issue. I haven't had time to make an exhaustive investigation since yesterday :/ – VivaLebowski Jan 15 at 15:02
  • @VivaLebowski Sounds like you had a pressure tank between where you turned off the cold water and where the cold water enters the water heater. I'm only aware of pressure tanks being on well systems, but perhaps you have something performing that function? – mike65535 Jan 17 at 17:08
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    @mike65535 I added pictures in the post above; don't think that's it. We have a normal setup to the best of my knowledge. There is absolutely nothing but copper pipe between the valve I turned and where it enters the top of the water heater. – VivaLebowski Jan 17 at 17:56

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