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After shutting off the water for several hours to install new sink and toilet now I have warm,almost hot water coming out of every faucet from the cold tap. It takes a full minute for it to get cold.

My plumber is stumped.

Can you help?

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    Hello, and welcome to Stack Exchange. Almost certainly the problem is the sink bridging the hot and cold lines. Test the temperature of the supply lines to the sink, and see if the cold side seems hot. – Daniel Griscom Dec 20 '18 at 3:40
  • One other posssiblity.... a mixing valve that someone touched. A plumber shouldnt be stumped by this issue. – noybman Dec 20 '18 at 3:55
  • Ohhh, one more possibility, The tank is on, cooking away, but the outlet is blocked, so as pressure builds, (even though it is moderate), it is forcing its way backwards, into the cold feed. – noybman Dec 20 '18 at 5:46
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I'm inclined to side with nobyman's shower mixing valve failure. A typical issue resulting from the water being turned back on all at once or too quickly and rupturing the rubber and plastic parts of this century's shower valves.

This is most common with ball valves. The best practice, especially with ball valves, is to just crack them to open and wait for the system to fill and return to silent before opening the valve the rest of the way.

Very rare but also happens with basement shut-offs that are quite a distance from the fixtures by creating a water hammer. This is one reason to have individual shut-offs for each and every fixture everywhere.

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Besides being broken as Iggy suggests, the (really tiny) valves to prevent backflow can get stuck open because of debris. I've had this happen several times here in my shower faucet, usually after other major work on the pipes, as in your case. A telltale sign is noise in the faucet when quickly shutting off cold or hot water at another faucet in the house. The noise is made by water traveling between the hot and cold sides to equalize pressure.

Failing that, it should be possible to find the guilty faucet by selectively closing valves until your problem goes away. And if your plumber is stumped by this, find a better plumber.

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