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I just moved into a house that was built in 2005. It has PEX run to each fixture, all through a single manifold (or at least, most through the manifold...). When we moved into the house, all seemed well.

After living here for a few weeks, however, we seem to have developed a crossover between the hot and cold. Specifically, in a certain 1/2 bath, the shower takes a long time to warm up, and never warms past lukewarm. Purely by accident, we discovered that if we open the hot water valve in the faucet in this same bathroom, the shower will become quite hot. Also, while the shower and sink are running in this state, cold water will run from the faucet instead of hot. This phenomenon does not seem to happen in the other bathroom we've tried (but the shower in that bathroom will get significantly warmer, so it is not necessary).

The only fixtures in the house (that I am aware of) that could be the source of the crossover are:

  • the bath/shower valves
  • the kitchen faucet
  • the washing machine

I have replaced the cartridges (Moen 1222) in all showers except one, which is currently under construction (rough plumbing has been done), no fixtures are in place. I have closed the valve under the kitchen faucet and behind the washing machine to no effect.

Where might the source of this crossover be, and how do I troubleshoot it?

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    I would think you will have anti-scald mixing valves. They mix cold with hot to reduce the chance of skin burning. When they are not working right, they can mix too much cold with hot or reduce hot a lot.
    – crip659
    Commented Nov 26 at 19:15
  • In the showers? They're just "normal" Moen 1222 valves. If there's another mixing valve somewhere, I don't know where it'd be. In the manifold, maybe? I'm measuring temperature in the dual-handle faucet nearest the water heater, so there's no mixing going on there.
    – Mark
    Commented Nov 26 at 19:30
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    Will need a modern plumber person to tell. I am from the time where you added cold water till the hot stop burning.
    – crip659
    Commented Nov 26 at 19:32
  • What's a shower doing in a 1/2 bath?
    – Joel Keene
    Commented Nov 27 at 7:12
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    @crip659 aka "tempering valve" <-- added for search engines.
    – Criggie
    Commented Nov 27 at 21:39

2 Answers 2

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The correct answer was suggested by my contractor's plumber: the crossover was coming from the rough-plumbed shower fixture. While they come with a "plug" installed, and this "plug" prevents water from leaving the fixture, this "plug" apparently does NOT prevent crossover. I did not know that, and neither did my contractor. Problem solved!

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The offending anti-scald valve(s) can be in any fixture that has one, routing cold into the hot line that feeds the shower.

It usually is NOT the particular fixture you note the problem with. But the sink that makes a change would be a good one to start your search at. However, there could be more than one fixture involved.

Diagnostics would be to run the other fixtures to get the supply piping hot and cold, then run the affected shower and find the fixture (or pipe at the manifold) where the hot pipe turns cold (or a cold pipe turns hot, but I'm expecting the former with your reported issue.)

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  • The sink that makes a difference (as all the sinks in the house, except the kitchen) is a two-valve sink, so there's no potential for crossover there.
    – Mark
    Commented Nov 27 at 0:20

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