Water hammer: still knocking after installing arrestors

[Clint Grammer]

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?

 

[Terry]

Do you have a PRV for the home? It may be that if it's only the home, all faucets and not irrigation.

Also, some knocking can occure with loose washers on shutoffs. But that would be just the fixture served.
How about the tub faucet there? Does that hammer? If so, maybe a new cartridge.

 

[Sponsor]

 

[Reach4]

I've been having a knocking problem ever since I bought the house a little over a year ago,

When does the knocking sound occur?

 

[Clint Grammer]

There is no prv, but I do feel like the pressure in the home is a little low. But the city did just tear down the closest water tower to my house that took a direct hit by an EF4 tornado 3 days after I closed on this house so I'm not sure if that is why the pressure is lower. All pipes in that area is tight and properly secured. The tub faucet is 20 years old, but it is tight with no movement. Do you think that since my pressure is lower than normal, that there might not be enough pressure for the arrestors to work properly?

 

[Clint Grammer]

When does the knocking sound occur?

It knocks when all faucets are turned off, and when some faucets are turned on

 

[Plumber69]

Narrow it down one faucet at a time hot and cold

 

[Jadnashua]

A hammer arrestor works only when you shut a valve off...the inertia of the moving water pushes on the piston or bladder in the arrestor instead of moving the pipe, or hammering it into some hard surface. In a dramatic situation you'd see the pipe literally jump when you shut something off. IF it comes on when you open a valve, I don't see how this is a typical water hammer.

To work best, arrestors need to be near the end of the line of the offending valve, i.e., near the valve, not somewhere in the middle of a run. Most common ones that can close fast enough and can create a water hammer are:
- washing machines
- some toilet fill valves
- ice makers
- humidifiers
- some shower valves

It has to be something that shuts off fast, not a typical valve with a volume control that is turned off, slowing the flow as you do it.