Pipes bang when turning water on

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by richardoc33, Jun 5, 2007.

  1. richardoc33

    richardoc33 New Member

    Jun 5, 2007
    I live in a 12 year old condo that has separate in-coming supply lines for the hot and cold water (units in the building share common water heaters). When I turn the cold water on at any of my sinks or flush the toilet there is a quite noticeable bang typically near the point where the line enters my unit. The hot water supply line does not bang. Each supply line coming in has:
    a. a shut-off valve, followed by
    b. a Watts pressure reducer valve (25AUB), followed by
    c. a vertical pipe for controlling water hammer.

    Wondering if a bad diaphragm or other internal part in the Watts valve could be the cause of the pipe bang?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Not sure, but I will tell you that unless those vertical pipes are a water hammer arrester (has a piston in it or a bladder), after a short time, they end up getting the air replaced with water and are no longer effective for the most part.

    You may want to check your water pressure. If you have a washing machine hookup, that is probably the easiest place to do it. You can buy a gauge for around $10 at a big box store that will screw onto the shutoffs there.

    My guess is that the pressure reduction valve on the cold is shot, but that's a guess. Pipes don't normally move (bang) when you open a valve, but when you shut things off. Missing or loose straps may help. Very high pressure can do strange things, so that is why I suggested just checking the pressure. PRV's go bad, but don't blame them until you know.

    Does this happen when you open any valve, or only certain ones? It could be as simple as loose washer in the valve.

    Last edited by a moderator: Feb 22, 2009
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  4. richardoc33

    richardoc33 New Member

    Jun 5, 2007
    OK, thanks I'll buy a pressure gauge and check it out. I get the banging when I turn on any sink cold faucet and when any toilet is flushed. I'm hoping the solution is to not have to cut holes in the walls to secure a pipe or pipes that may be hitting wall or ceiling frames.
  5. richardoc33

    richardoc33 New Member

    Jun 5, 2007
    Jim, I bought a water pressure gauge and hooked it up as you suggested on the washing machine cold water hookup. With all faucets turned off the gauge reads close to 90 PSI. When I turn on any of my cold water faucets or flush a toilet the gauge reads about 50 PSI. I turned the adjusting screw on the Pressure Reducing Valve to lower the pressure. This had no effect on the water pressure with all faucets turned off (gauge continued to read about 90 PSI), but the gauge read a much lower PSI when I turned on any of my cold water faucets or flushed a toilet. Wondering if this indicates a correctly functioning PRV or not?

  6. Cass

    Cass Plumber

    Nov 12, 2005
    If you have a PRV and the pressure reading is 90 PSI the gauge is bad and needs to be replaced. Most are adjustable up to 75 PSI. but not over.

    That is why when you tried to adjust it down nothing happened.

    While you replacing the PRV you should also replace the expansion tank, if you don't have one you need to install one on the cold water line.

    They are normaly found above / near the water heater.
    Last edited: Jun 14, 2007
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