If the pressure is to high at the faucet creating back pressure and I turn down the water at the main water shut off, would that reduce the the pressure enough to stop the water from coming out the shower. I thought that would be the case but I was wrong and water still came out the shower head when filling the tub.

Question: If reducing the pressure there doesn't help then how much help would replacing the Pex line for copper be?

Can anyone she'd some light on my situation. I might add that there is a considerable amount of water coming from the shower head. The shower head mixer and faucet are Moen products

  • Do the instructions for the mixing valve say what type of piping to use? Do you have the simple kind of diverter which creates a blockage in the tub spout (usually pull up on a rod to pull blockage into place? Commented Mar 22, 2018 at 23:03
  • what kind of shower / tub faucet assembly do you have ? Single handle with pull ? Can you take picture or post model of faucet .. What it sounds like to me is the unit was somehow connected wrong ..(shower and faucet lines as incoming supply and the incoming supply lines to the shower / tub )???
    – Ken
    Commented Mar 23, 2018 at 7:06
  • I will take some pic's a little later today and double check if its connected wrong as you say but for now it's moen with the single pull handle and yes I believe it's the simple diverted as well Commented Mar 23, 2018 at 12:48
  • There are on-off valves that one can install just before the shower head. This would stop water coming out the shower when you want water for the tub. The purpose of these valves is for water conservation during showering, but it would work to solve your problem. signaturehardware.com/… Commented Mar 23, 2018 at 14:40

2 Answers 2


Sounds like you used PEX to run from the mixer valve to the tub spout. Replace the PEX that goes from the mixer to the spout with copper. You are getting a bypass to the shower because the PEX from the mixer to the spout cannot handle the flow without backing up to the shower head. PEX has a slightly smaller i.d. than copper.


What I did when I had this problem was got a wider pipe for the little run between the valve and the faucet, that kept the pressure from building all the way back up to the showerhead

  • 1
    Welcome to Home Improvement. Can you tell the asker how much wider? Was going from 1/2" to 3/4" sufficient, or did you need to go to 1" (or bigger)? Adding some details will make this a much more useful answer.
    – FreeMan
    Commented Feb 25, 2021 at 12:41
  • IIRC 3/4 PEX inner diameter is larger than 1/2 copper ID, so that probably would be enough, assuming the existing PEX is 1/2. Commented Feb 26, 2021 at 3:35
  • I was trying to remember while I posted but it’s been almost 10 years since that project and I didn’t want to assert the wrong size. As a realistic guess, I think we used 3/4” copper because it was before I discovered the joy of PEX.
    – Pete
    Commented Feb 26, 2021 at 15:42

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