Delayed Single Knock When Shower Is Turned-Off

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by AlanDFW, Nov 11, 2019.

  1. AlanDFW

    AlanDFW New Member

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    I have been searching through this forum in hopes of finding and answer, but do not seem to be able to find something similar.

    I recently had a new shower installed using the Moen Posi-temp valve. 10-15 minutes after I turn off the shower, there is a single knock/bang noise that comes from the shower. I have trouble shot the problem like you would for water hammer, but that has not yielded any luck. What baffles me (and the plumber who did the install) is the bang noise is delayed by 10-15 minutes. I do not experience this issue with any other faucet in the house, so it makes me wonder if it is related to the valve (I didn't have this issue prior to the remodel, but it was a different valve). Could it be a faulty valve cartridge?

    Appreciate any and all suggestions.
     
  2. WorthFlorida

    WorthFlorida The wife is still training me.

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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Is this a tub/shower, or a dedicated shower? If it's a tub/shower, it could be the divertor releasing. Depending on the design, that doesn't always happen immediately.
     
  5. AlanDFW

    AlanDFW New Member

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    It is a shower only. Sound is a single “thonk” type noise.

    Happens every time the shower is used. We shower at night, so always hear it as we are laying in bed at night. Just a single clank, but can’t seem to make it go away.

    We had the plumber back out, wall was opened up, everything is secured. Shower valve cartridge was replaced. Still the thonk noise about 15 minutes after the shower is turned off. We’re all stumped.
     
  6. WorthFlorida

    WorthFlorida The wife is still training me.

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    Give this a try, remove the shower head and leave the shower arm intact. Run the water, shut it off and wait. Try keeping your hand on the shower handle so when the noise occurs, you might get a sense of exactly where the clunk noise is coming from. The valve body or elsewhere.

    Are there any hammer arrestors installed at the shower valve body? If you can make this occur regularly, shut off all valves at every faucet in the home including the washing machine. Noise from fixtures can travel very easily throughout the home through the plumbing.
     
  7. mliu

    mliu Active Member

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    I believe your problem is being caused by thermal expansion. As you use the shower, the pipe to the showerhead heats up and expands. After the shower, it slowly cools down and contracts. If the pipe is hanging-up on a support or something else in the wall, then it will develop tension which will increase as the pipe cools. Eventually (~10-15 minutes after the shower), the tension is strong enough to overcome whatever is hanging up the pipe. Then it releases and the pipe springs back with a "thunk."

    You can test this by running only cold water in your shower. If I'm correct, there will be no "thunk" after you shut it off.

    You can also do another test: after you finish your shower, turn off the hot water first and allow some cold water to run. You may or may not get a "thunk" right away with the cold water (rapid cooling may allow the pipe to move without developing tension). But the cold water in the pipe after the shower should prevent the "thunk" 10-15 minutes later.

    If this is indeed the cause, the only solution will be to open the wall and find out what is causing the pipe to hang-up.
     
    WorthFlorida likes this.
  8. WorthFlorida

    WorthFlorida The wife is still training me.

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    Is it copper pipe in your home? Great point mliu.
     
    Last edited: Feb 16, 2020
  9. Terry

    Terry Administrator Staff Member

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    Pretty common with an ABS drain for a shower that passes through a tight or slightly crooked hole. The pipe expands when heated by the warm shower water, and then contracts as it cools. The knock happens when the pipe releases from the wood around it and pulls back. The solution is to enlarge the hole around the drain line, or to quit listening for it.
     
  10. AlanDFW

    AlanDFW New Member

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    It is copper pipe in the wall. Sound comes from the area where the shower arm connects at a 90 degree turn with the piping in the wall and heads towards the valve.

    My guess was also expansion, but when the wall was opened up, they said it wasn’t touching anything.
     
  11. plumber69

    plumber69 In the Trades

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    You are not hearing the diverter reseting its self are you?
     
  12. mliu

    mliu Active Member

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    Show us photos of the piping with the wall open.

    Did you test the shower while the wall was open?

    Have you performed the tests I described above?
     
  13. AlanDFW

    AlanDFW New Member

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    Mliu, I will try the cold water after a shower test tonight. I do know that if ran on cold, or ran just a short time, no noise is made which is why I believe your thermal expansion theory.

    The plumber was able to make the noise with the wall open, but was baffled by what it was. Wall is currently sealed back up. I looked through the hole where the shower head comes out and can see that the L joint it connects to has two screws securing it to a stud (not a plumber, so no idea if normal). There was some drywall touching the L joint, so I knocked that all off.
     
  14. Reach4

    Reach4 Well-Known Member

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    That fitting you describe is called a drop ear elbow.

    Are you saying that the noise is gone now?
     
  15. mliu

    mliu Active Member

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    Let me see if I understand the timeline:

    1. After the shower was initially installed and finished, you discovered this sound.

    2. You had the plumber come back and open the wall.

    3. The plumber replicated the sound with the wall open, but was baffled about the cause.

    4. The plumber took no corrective action and sealed the wall back up.

    5. The sound continues.

    Correct?
     
    Last edited: Feb 19, 2020
  16. AlanDFW

    AlanDFW New Member

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    Nov 11, 2019
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    Noise is still there.

    mliu, number 4 should read the “plumber” thought he figure out the reason for the sound, corrected and resealed wall. He was wrong and sound remains. He’s the guy the contractor who redid the bathroom uses, so not sure of his qualifications.

    Might call a real plumber or just open the wall myself and poke around. It’s definitely thermal related as the cold water test sped up the knock sound.
     
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