Knocking when cold supply turned on or off

24 Aug 2012
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United Kingdom
Hi, I'm new to this forum and have a problem with a banging / knocking whenever the cold water supply is turned on or off. This has been noticeable for a few weeks but seems to have got worse recently.
Having read up a bit online I guess that this is "water hammer" but it is not just on one tap - it happens with all taps, toilets, washer etc. I understand that this is a pressure problem and have turned the stopcock in the house down to try to get rid but to no avail. The water company have checked the supply and have said pressures are normal.
Any ideas what could be the cause of this and how do I resolve it? I'm thinking maybe the stopcock is faulty, although it does shut off the supply as normal.
Any advice would be appreciated.


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No problems with the supply, just the age old problem of your pipes not being fully clipped to walls or joists etc.
Its best to lift floorboards in the vicinity of the clunk, if you possibly can, to see whats what.
John :)
Thanks John.

Not easy - modern house with all pipes behind plasterboard or tiled walls and we have weyrock floors.

Difficult to say where the noise is coming from but as it happens with every cold tap and WC in the house is it likely to be an unsecured pipe? For example I also get the noise when the washing machine is drawing water, and this is in the utility room outside the main part of the house. The supply to that room comes straight off the main in the kitchen. This is what leads me to suspect a problem at the stopcock. Someone has suggested fitting a pressure reducing valve just downstream of the stopcock, but I don't see how this will help as I have the stopcock turned right down to reduce pressure.

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It certainly is an unsecured pipe, and the pressure wave runs down from any tap until the unsecured pipe is reached, which obliges with a clunk.
You'll find turning one tap on, and leaving it running whilst another tap is used will either stop the noise altogether or significantly reduce it.....until the running tap is turned off, naturally. If that tap is turned off really slowly the hammer will stop, but a washing machine or toilet doesn't know this!
Turning the stopcock back may help to some degree but this will reduce the flow rather than the pressure.
A shock arrestor could help, but it does depend on where it is fixed in the house.
John :)
Thanks again John

Your explanation certainly fits the symptoms. Trouble is where to start, as I said before, all pipes are hidden behind tiles or plasterboard walls, or beneath weyrock floors.
I'm worried about the implications of doing nothing as I've read that if not sorted the vibration can result in pipes bursting at joints.

The easiest thing for you to do Martin is to fit a shock arrestor at some convenient spot, as close to the main stopcock as possible, really.
I've fitted one or two Toolstation ones to certain effect - the outcome was better but not perfect. Sorry I can't recommend any particular product!
Smaller shock arrestors are available to replace the like of washing machine / dishwasher water connectors - they can't do any harm and may be worth considering.
John :)
Thanks John

Your suggestion ties in with what the lad from Northumbrian Water was suggesting. I'll get that done and post the result on here in due course.

Finally resolved this problem.

First off fitted a new stopcock. This appeared to solve the problem, as the banging stopped ........... For about two days.

So next step was a pressure reducing valve immediately downstream of the stopcock, together with a shock arrester. Under the sink now looks like a plumbing exhibition, but the noise has stopped - hopefully for good.

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