We didn't use to get pipes knocking.
We run a Glow-worm Swiftflow combi boiler.
Suddenly now, when we turn the shower off (running off the combi boiler), we
get a loud noise (maybe a knock).
Can anyone explain what may be the cause of this.
If you have lots of water at high pressure travelling fast through a pipe
and then stop it dead, it has nowhere to go and sends a shock wave through
the system in an attempt to dissipate the energy. It solved by installing a
"shock arrestor" near the offending tap/valve. In the olden days, a short
stub of pipe was left, full of air for the same reason. A shock arrestor is
more reliable and hygenic. It is basically a tiny miniature expansion
vessel, (sometimes spring loaded instead of using a air pressurised
It may have got worse recently because pressure is greater, a pipe clip has
given up the ghost (after physically preventing the knock for some years),
or someone with a faster technique has started using the tap/valve. A
particular cause of new knocks is the installation of quarter turn
taps/valves, which close much faster than a twist your arm off type.
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