Water hammer when testing a new shower install with no shower head

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, back 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.

Mark

If I'm not mistaken, most valves mention not using pex from the valve to the shower head. You may be understanding why they do not recommend it.

For starters I would reduce the water pressure. What rough in valve is installed? Some off brand shower cartridges can have noisy pressure balancing spool.

It is a Kohler:

K-8304-KS-NA

With no shower head on it you can get a lot of water flow at 80 PSI. I'm going to add some hammer arrestors at the valve inlets and I'm also going to install a cheap shower head to see what happens. I know if I put the end of the tube in a bucket and just place my finger over the tube end and apply a slight amount of pressure the hammer does not happen. I assume this is because that added just enough restriction to cut back the very fast flow of water. I'm hoping the shower head will slow the flow down a bit.

Mark

Turn the water on slower and turn it off slower. Easy……