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The issue I am having is that very often, the water in the cold line from my taps is cold for a few seconds when I turn it on, and then it warms up almost immediately and stays warm.

Last year, I was having the same symptoms. I had the plumber out (builder's warranty was still in place), who discovered hot water was flowing backward through the cold water inlet of the hot water heater. He said the backflow preventer in it must have failed, so he installed a check valve on the cold water inlet right past where the expansion tank and shutoff valve are.

Fast forward to a couple of months ago, and the problem has been slowly coming back again. Now it's worse than it ever was. Sometimes the water seems cold and stays that way, but more often than not, it starts cold and then gets warmer.

Initially, I thought the check valve the plumber installed must not be working. There is a shutoff valve on the cold water inlet right above the expansion tank and below the check valve, so as a test, I turned that off completely, and I turned off power to the water heater.

Then I went around testing faucets. As soon as I opened the bathtub mixing valve just a little bit (trying to get only cold water to come out), the water got really warm. I let it run for a bit, and it stayed warm. I turned it so that barely any water was coming out, and it still stayed warm. Then I noticed the cold taps nearby all got warm quickly and seemed to stay warm.

Does this mean I have a bad mixing valve at the tub faucet? I'm going to try and shut off the hot side if it's possible to do so at the faucet so that I can see if the problem stops.

If it does turn out that's the issue, it seems strange that my house (built in 2020) would run into two seemingly unrelated plumbing failures that both caused the same behavior (hot and cold water mixing). Could that possibly be a sign that there is too much hot water pressure or some other larger issue, or am I just reading too much into two separate events?

Update: I forgot to mention I was doing a load of laundry on the tap-cold setting on my washer at the time. After re-testing everything, this time the tub faucet stayed cold when it was the only thing on, but as soon as I turned on the washing machine to cold, warm water started coming out of the washing machine as well as nearby faucets.

I'm thinking the bathtub water was warm because of a possible washing machine issue, so I'm attempting to leave the hot water shut off at the washing machine and see if the problem is affected in any way.

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  • Sounds like you may have figured it out with the washing machine... Are you just looking for other ideas?
    – JPhi1618
    Sep 20 at 20:45
  • I don't think I've solved it yet. I turned off the hot water at the washing machine, then ran the cold water for several minutes on some other taps, and the water is still not as cold as it should be. I guess any of my mixing valves could be the culprit? And more than that, my main question from my OP was whether multiple events that cause this problem is a sign of a larger problem with my hot water system.
    – Ben
    Sep 20 at 21:24
  • What kind of water heater do you have? Is there a chance there could be some bypass valve that is open? It would have to be a mixing valve that also wasn't outputting any water, so the hot and cold are joined but there is no output. Also, it's not just water pipes running through the attic so they get warm right? The water stays warm even after "stagnate" water in the pipes would have been used up?
    – JPhi1618
    Sep 20 at 21:33
  • It's an AO Smith electric water heater. Would shutting off the cold water supply to the water heater rule a bypass valve out, or could that still be related? All of my faucets and both showers use single handle mixing valves. If the issue persists, should I shut the water off to all of them and test them one at a time?
    – Ben
    Sep 20 at 22:36
  • No pipes running in the attic, they all come up from the basement to the fixtures. I don't know exactly how long it takes for stagnant water to leave the cold water pipes, but I opened the kitchen and bathroom faucets for about 5 minutes, and then I've been going back and opening them again to check regularly. It does seem like it's colder than it has been for a long time, although the hot water line is still colder than the cold water line when I first turn it on which seems strange.
    – Ben
    Sep 20 at 22:41

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