Water hammer in new shower when shutting valve off fast

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Grantham, NH
I'm not a professional plumber. I'm remodeling a bathroom with a tiled walk-in shower. The temp valve is located on the back shower wall about 50" from where a Kohler Hydro-Rail will be located. There is a fairly long run of tubing from the valve, bacl down to the basement, 50" under the floor joist and then back up the shower head wall, probably 150" - 160". At the moment the walls are still open and I have a section of pex tubing and fitting threaded onto the shower head drop ear just to test the base for leaks and make sure the pan is draining properly. I do have fairly high water pressure, 80 PSI, by using a pressure booster pump since the water pressure coming into the house is poor, 30 PSI.

All the plumping for the shower is 1/2" pex and it is secured to the studs and floor joist in numerous locations.

I noticed that when testing the pan for leaks I get a bad water hammer on the shower head tubing if I shut the water off fast. I plan to install some arrestors on the supply lines near the valve but what is odd is the hammer is all on the output side and mostly toward the end of the shower head run. The end tubing is just wide open. Why would the hammer be there and not on the supply side where the valve closes?

I have not tried this with a real showerhead on the drop ear. I can't temporarily try it with Hydro-rail, it is kind of a fussy install.

Any thoughts I way this is happening? Will arrestors on the supply inlets help? I'd like to try and make sure I don't have a problem before closing in the walls and tiling the shower.


Tuttles Revenge

In the Trades
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"Water hammer" is caused when the momentum of the water travelling through the pipes is suddenly stopped. It has to go somewhere, so it moves the pipe in most cases.

When you say Temp valve. Does that refer to a temperture or temporary valve? If its the shower rough in valve, does it have the cartridge installed? You don't need to install the entire hydrorail to test it with the shower head. A nipple with a 90 and another nipple can simulate the shower ell and you can thread on your shower head to that with a bucket to catch water if you're concerned with splashing. I bet that your flow rate with a showerhead reduces the velocity of the water to a point where this becomes a non issue. But.. best to get this solved before covering.
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