Water hammer mystery

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by dang5001, Jan 18, 2014.

  1. dang5001

    dang5001 New Member

    Messages:
    5
    Location:
    NC
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
  3. dang5001

    dang5001 New Member

    Messages:
    5
    Location:
    NC
    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,311
    Location:
    Cave Creek, Arizona
    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.
  5. dang5001

    dang5001 New Member

    Messages:
    5
    Location:
    NC
    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. Smooky

    Smooky Member

    Messages:
    569
    Location:
    NC
    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. dang5001

    dang5001 New Member

    Messages:
    5
    Location:
    NC
    What did they used to do before expansion tanks?
  8. Smooky

    Smooky Member

    Messages:
    569
    Location:
    NC
  9. dang5001

    dang5001 New Member

    Messages:
    5
    Location:
    NC
    Thanks for the advise. I'll probably just install an expansion tank just to be on the safe side.
  10. Reach4

    Reach4 Active Member

    Messages:
    1,936
    Location:
    IL
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
Similar Threads: Water hammer
Forum Title Date
Plumbing Forum, Professional & DIY Advice, Tips & All of a Sudden - Water Hammer? Jun 1, 2014
Plumbing Forum, Professional & DIY Advice, Tips & installing water hammer arrestors May 26, 2014
Plumbing Forum, Professional & DIY Advice, Tips & Humidifier water hammer Mar 2, 2014
Plumbing Forum, Professional & DIY Advice, Tips & Do water hammer arrestors wear out? Feb 23, 2014
Plumbing Forum, Professional & DIY Advice, Tips & After replacing shower valve getting water hammer. Feb 22, 2014

Share This Page