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Thread: Water hammer mystery

  1. #1
    DIY Junior Member dang5001's Avatar
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    Default Water hammer mystery

    I apologize for the long post in advance.

    About 2 months ago I started have water hammer every time I flush a toilet and when using a new washing machine (new washing machine was installed after Christmas). Installed the arrestors at the washing machine, still clunking in the pipes. Had the Town come check pressure at the tap, it was at 85 psi, so, yesterday I installed a PRV, pressure now at 60 psi. Still have water hammer. Sounded like it was in the lines going to the shower in my hall. I had previously installed ball valves on all my hot and cold water lines in the basement going to my shower faucets. So, I stopped the flow, both hot and cold to the shower in question, flushed the toilets and the hammer stopped. Opened the hot kept the cold closed, no hammer. Opened valve to cold, hammer came back. It only happens with the cold water valve open to that particular shower. I then installed an in line arrestor on the cold line behind the faucet in the wall, still have the water hammer. I even changed out the ball cocks in my toilets, no change.

    My questions are:
    Would an expansion tank help? Could the problem be in the valve stem at the shower faucet? I am lost at this point. Any help/advice will be greatly appreciated.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    First, once you add the PRV, you must install an expansion tank, or unless you have a leak somewhere, your T&P valve on the water heater will weep when it is reheating the water you used. The addition of an ET shouldn't stop the banging, but it is a possibility - they are not designed for that purpose, but it can offer a little protection.

    It sounds like at least one pipe is not supported well, and when water shuts off, it moves the pipe and it hits something, which is what you are hearing. Long-term, that can potentially wear a hole in it, depending on what it is rubbing against or hitting.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member dang5001's Avatar
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    Quote Originally Posted by jadnashua View Post
    First, once you add the PRV, you must install an expansion tank, or unless you have a leak somewhere, your T&P valve on the water heater will weep when it is reheating the water you used. The addition of an ET shouldn't stop the banging, but it is a possibility - they are not designed for that purpose, but it can offer a little protection.

    It sounds like at least one pipe is not supported well, and when water shuts off, it moves the pipe and it hits something, which is what you are hearing. Long-term, that can potentially wear a hole in it, depending on what it is rubbing against or hitting.
    I don't have a leak and all of the pipes are secure. As I said, when I shut the ball valve to the cold water line on the hall shower and flush, the knocking stops. Open the valve and it knocks. I'm going to add an expansion tank, but I don't think that will solve the problem. What I can't figure out is the fact it will knock with that one cold water valve is open and not when it is closed. Do you think it could have something to do with the cartridge in the shower faucet? I would have checked the cartridge but the set screw is stripped on the handle and I need to drill it out.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    It could be the balancing spool in that shower reacting to a drop in the cold water pressure when you flush the toilet or use cold water. If so, then your real challenge is to find out why you are getting that pressure drop.
    Licensed residential and commercial plumber

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    DIY Junior Member dang5001's Avatar
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    hj,

    I changed out the cartridge and guess what, no more hammer. I wished I had come to this site before I went through my process of elimination. I want to thank you and I have one more question. Is it really that necessary to add an expansion tank now that I have installed the PRV?

    Again, thanks for the advice.

  6. #6
    DIY Senior Member Smooky's Avatar
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    Most PRVs are one way valves. Your water heater will cause the heated water to expand. If there is no place for it to go your T&P valve on the hot water heater will open and let water drip out. Sometimes the T&P valve gets stuck open etc or it may be in an area where you do not want it to drip. The thermal expansion tank is the way to go if you have a closed system.

  7. #7
    DIY Junior Member dang5001's Avatar
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    What did they used to do before expansion tanks?

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    DIY Senior Member Smooky's Avatar
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    Some people use a pressure relief valve. It drips water out instead of using an expansion tank.
    http://www.homedepot.com/p/Parts20-P...60LF/202076781

    I the past pressure backed up into the system. Also some pressure reducing valves have a bypass that opens if the pressure is higher than the inlet pressure.

  9. #9
    DIY Junior Member dang5001's Avatar
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    Thanks for the advise. I'll probably just install an expansion tank just to be on the safe side.

  10. #10
    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by dang5001 View Post
    What did they used to do before expansion tanks?
    I don't think it was so common in the past to deliver more than 90 PSI water to houses. I may be wrong about that.

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    When I was growing up, our house was maybe a 1/4-mile from the spring fed town water supply...without a PRV, opening the valve could shoot the glass out of your hand if you were not careful...a PRV solved that issue...that's more than 50-years ago. Each situation differs. Get into hilly country, and the water pressure can vary considerably depending on whether you're on the top or the bottom of the hill.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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