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Noisy pipes when water flows


Here is what is happening. When I run the HOT water, not the cold, in certain fixtures a very loud banging occurs and it sound like there is a loose bib washer. I'm sure you've heard that type of sound I'm describing. It only happens in faucets with no restriction such as my washing machine, master tub filler and a wide spread water fall lavatory faucet. All other fixtures are fine and I have only washer less faucets in my home. I have a Rennai tankless gas heater outside the house which has freeze protection. With freeze protection, there are two check valves installed inside the house and a solenoid valve as well. I had the two check valves replaced thinking that would solve the problem. It did not. I checked my incoming water pressure and it was 65 psi. I went ahead and changed the Pressure Reducing Valve which is now set at 50 psi. That too did not work. I also cleaned the incoming water filter on the heater with no luck.
I now have a Rennai service tech coming out next week to check the heater which is about eight years old. I have called Rennai and they are no help. They never heard of this problem with their heaters. I do not have a recirculator on the unit.

My question is what would cause water to bang when it is flowing? I am familiar with water hammer and installing water hammer arrestors when water is stopped quickly but this is new to me.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    It appears to me maybe that pressure is building up between a check valve and the fixture probably from the water heating up.
    Also what is the solenoid valve for?
    Can you draw a diagram of your domestic piping with all the components up to and into the boiler?

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  • firebuff
    firebuff Member Posts: 4
    Hi Steve,
    The banging begins as soon as I open the faucet of the master tub or the waterfall lavatory faucet or use the washing machine.
    (None of these have restrictors)
    It begins banging even before the gas is fired up in the tankless heater. The solenoid valve which is normally closed, opens when there is a loss of electricity and drains the heater to prevent freezing. I'm a retired plumber and I've never run into this problem before. And to top it off, it's my house that this is happening in. I half closed the valve for the hot water under the waterfall faucet and that stopped the banging simply by decreasing the flow. Everything has worked fine since the unit was installed 7 years ago.
    Thanks for your help.
    JUGHNE Member Posts: 11,074
    Would you have air chamber/cushions plumbed inside the walls?
    If so they could be water logged.
    I had them and would have to shut off the house and drain down the water to reestablish the air "head" in the chambers.
    That small amount of air did make the difference when the ice machine shut off.
    I realize your issue is when you get large flow.
    Worth a try and free.
  • hot_rod
    hot_rod Member Posts: 22,244
    Is there a flow restrictor of some sort in the tankless? that could be banging around at high flow?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • firebuff
    firebuff Member Posts: 4
    I thought of that and I'm not sure. Nothing in the literature about that but I will call the Rinnai rep and see.
    John in Georgia
  • Zman
    Zman Member Posts: 7,572
    If you have a check valve on the house, you should have a Domestic expansion tank to prevent over pressure and hammering due to thermal expansion
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,855
    edited August 2018
    I'm not real familiar with tankless heaters but isn't there a restrictor and screen on the cold into the tank? How's the voltage to the flow sensor when when you open a hot tap? Does trial for ignition start immediately upon opening a hot tap?
    Its probably safe to say you can adjust the pressure back to 65 psi. Like @Zman said, a domestic extrol charged to 65 psi after the PRV if one is not already there.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,805
    Charge the thermal expansion tank to the same pressure as the water pressure ...

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  • firebuff
    firebuff Member Posts: 4
    Big Ed wins the prize. We charged the expansion tank to the incoming pressure and that did the trick.
    Thanks everyone for your help.
    Fire buff