Water hammer: still knocking after installing arrestors

[cdgrammer]

I've been*having a knocking problem ever since I bought the*house a little over a year ago, and I have some Sheetrock out and figured to fix the problem. I installed 2 water hammer arrestors where the knocking is coming from (one on cold line, one on hot), the knocking stopped for a couple a couple of days, then returned. It*will knock with all faucets inside the house, but will not knock with irrigation system. Pressure inside the house is 54 Psi. Any ideas how to get the knocking to stop before the sheetrock goes back in?

 

[Mr_David]

The arrestors you added are kind of useless.

The 2 risers above the valve will work better than those things you added.
If you replace them with 3/4" pipe, they will hold more air and work better as shock absorbers.
drain the system. you need to allow the water to drain from the risers above the stub valve.

If all the fixtures knock the I would look for the problem common to all ,like the water service coming into the house.
Any check valves on the system?
Recirc lines for hot water often have check valves.

I had one a couple weeks ago, recirc pump was in backwards and it made a check valve rattle.

Your system is in the slab. Hammer is not from loose pipes. Got to be a defective valve

 

[Debo22]

Grab those pipes and shake them, if any are loose secure as needed