Hi, I have a 40 yr. old home. I never had any problems until I remodeled my master bath. I took out the bathtub and created a shower with new fixtures. I also replaced the sink and faucets. Since then, both upstairs bathrooms get hot water out of the cold water faucet. I turn on the cold water faucet and it runs cold for about 30 seconds, then turns very hot, then turns back to cold. I replaced the old piping with the new pec flexable pipes only in the master bath, but this is effecting the guest bath too. Please help!

Did you happen to run the new lines tight together any place along the run?

No, only lines installed was under the shower bed and up the shower wall. They weren't touching as I can remember.

How about any added under floor heat or auxiliary heat source to the bathroom in the remodel? Like Joe's suggestion, one of the pex cold lines could be bumping up or touching some sort of heat source.

Hi,
No floor heat. This hot water is also in the hall bathroom???? Also in the toilet. That has never happened before the replacement of the pipes in the master bath. It makes me think that there is back flow or a problem below the second level floor.

A simple test to see which faucet is causing this is to shut the hot and cold supplys off under the sink one at a time and test the other faucets to see it they work correctly. Make sure to run enough water out of the cold faucet to make sure it was not hot water still in the line.
The hot can not "cross" over with the line shut.

This message has been edited. Last edited by: joecaption,

A dozen years ago on a US army post in S. Korea a toilet sprung a leak. Korean workers decided to replace it and re-plumb which resulted in a hot water pipe getting connected to the toilet. The resulting abomination was short-lived, but for a while a few soldiers had the experience of a lifetime. Y'all can imaging the details.

i know this is a old thread.
gonna answer anyway
shut off water at water heater
turn on cold water at each fixture, one at a time
and turn off. see if water comes out the cold side or not
if NO water comes out of the right side when turned on
there is the problem
if its a moen valve, pull the cartridge and flip 180 degrees, reinstall

Even though it's an old thread, people that come up with the same problem can see this and get help.

The best way to troubleshoot this problem is to start at the source, the hot water tank, and go from there. A line has to be crossed somewhere. Hot water just doesn't materialize out of nowhere for no reason.