Water hammer

[XanderHarris]

I have water hammer, turning off my stop tap doesn't stop it. But turning off my external one does. This suggests the problem is with the external stop tap?

 

[blup]

Does the hammer occur when a particular tap or valve turns off or closes in your house? It’s possible that this sets up a pressure wave that travels to a loose part in the main service stop cock which creates the hammer.. You could try partially closing it to see if that makes a difference.

 

[tbcnsg]

If shutting off your internal stop tap doesn’t change anything, but closing the external one kills the hammer straight away, it usually means the issue is somewhere between the external stop tap and your internal plumbing.
Quite common for an old external stop tap to have a loose or worn jumper/washer that chatters when the flow starts or stops. They can vibrate like mad and send the noise right into the house.
Another possibility is that the pipework just after the external stop tap isn’t clipped properly, so when the tap partially restricts flow, it creates a pressure spike and the loose pipe amplifies it.
Easiest check: have someone turn a tap on/off while you stand at the external stop tap with a hand on it - if you can feel it rattling, there’s your culprit.
If it is the external tap, replacing the insert or the whole tap usually sorts it out.

 

[XanderHarris]

Thanks is a new external stop tap a plasson one. Then mdpe pipe into the building

 

[XanderHarris]

The external pipework is all mdpe. It's on a shared main. I wonderd if the problem could be coming from up or downstream.