Copper pipes making knocking sound when I turn the water off

Question

I did bathroom reno and replaced bathtub and shower faucets locations and extended copper pipes a little. That's not the first time I weld copper pipes and I didn't have any problems in past. What I noticed Is that when I turn off the water (cold or hot) I hear knocking like pipes knock on something. I examined all the places where I changed the plumbing and everything looks secure and doesn't touch anything. But I feel that knocking inside the pipes. It's like a pressure issue inside.

Probably it was before my work. I recently bought the house but it annoys me. Should I be concerned about it and can that knocking damage my pluming?

Answer 1

That's called "water hammer" and comes about when a valve closes quickly and the water running through the pipe kind of "bounces" back in the opposite direction. That creates a pressure wave that vibrates or bounces the pipes and you get an audible sound when the "dancing pipes" bang against stuff.

It usually doesn't do harm to the plumbing unless it's really strong or especially frequent. In which cases it can cause leaks and so forth in the pipes.

It's worth fixing because it's "fingernails on a chalkboard" annoying.

Plumbers install air chambers above where water lines for sinks, bathtubs, etc. are roughed in. This serves to provide a shock absorber for the "bouncing water".

If the walls are still open you could add this fairly easily.

The easiest way to "fix" it if the walls are closed up is to install water hammer arrestors at the point where your faucets attach to the supply lines or elsewhere in the supply lines. They consist of a cylinder with a piston kind of thing that absorbs the pressure wave when the water is redirected back on itself.

Illustration of water hammer arrestor (From Family Handyman - via Pinterest)