UK Plumber Forum

[ian.m.henderson]

Hi Guys, this is a weird one.
Occasionally when hot water is drawn in the bathroom, a loud knocking noise is heard in the pipework.
It will only stop if the cold tap in the kitchen is turned on for a few seconds.
The noise never occurs if cold water is drawn in the bathroom.
It does appear that it will only happen if the cylinder has hot water.

Any ideas where I should look

thanks

[REDSAW]

sounds like you have an open vented system. the noise will be coming from your tank valve when it replenishes the hot water cistern. to test tie up the float valve then run hot water , then release if after no noise to prove it. then service the valve.
it can also be to high a mains pressure causing hammer or a loose pipe-clips missing which is harder to find.