Can anyone explain this? (Knocking pipe)

24 Dec 2004
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United Kingdom
For ages I've had a single abrupt knock when the washer closes off after letting in cold water or if the cold tap is closed off suddenly in the downstairs bathroom (the washer is in a cupboard adjacent to this bathroom and the water supply is tee'd off from under the sink).

My stop tap is in the garage with a garden tap for a hosepipe above it before the cold water supply enters into the house (integral garage).

Anyway I noted today that if the hose reel is attatched (and the tap to it turned on) the loud knock disappears on shutting off the tap. If I close the tap to the hose reel the knock returns on closing off the tap.

What's happening? (and does it point to some possible solution?? - I don't like leaving the hosereel attatched in the garage and turned on, although it seems a neat solution to something that's irked me for a while).

All advice appreciated.
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This caused by kinetic pressure, try turning the water pressure down at the stop cock.
Water shock /hammer - tho` only one hammer blow ;) Your hose is acting as a shock arrestor you can buy a proper one from a plumbers merchant :idea:
The shock dissipates in the hose then? I'll have a read about a "shock arrestor". It sounds as though the noise comes from under the bathroom floor.
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water hammer noise will travel all round your property.

You will have a loose/broken jumper washer in a mains stop cock or possibly too many stop cocks within a relatively short distance. It's better to solve the problem rather than attempt to mask it with a shock arrestor.
Some new water meter control valves have a non-return valve in which will highlight any loose jumpers.
This caused by kinetic pressure, try turning the water pressure down at the stop cock.
Partly closing the stop tap does nothing for the static pressure, it just limits the flow to the whole house. It may do the job - until the increased velocity through the stop tap erodes the valve seat and washer so it won't close when you need it to.

First task should be attending to any loose or poorly supported pipework (under the bathroom floor?) then maybe consider fitting a flow restrictor at the washing machine hose before moving on to shock arrestors etc.
I agree, the reason that I suggested this was to enable "ajdoc"to quickly identify if it was high water pressure that was caused this problem.

incidently I forgot to ask "ajdoc"if the washing machine was connected with the proper length washing machine hose? as this usually stops the water hammer.
Is the washing machine connected with the proper length of washing machine hose?. As this should absorb the sudden increase in water pressure.
Thanks for all your help and advice, it's much appreciated. Well the new washing machine hose is a so called "pex" hose which is meant to be a bit stiffer than the usual hoses (I spent quite some cash and bought an "ISE 10" washer, if anyone's heard about them).

The noise does seem to come from under the downstairs bathroom floor, just have to see if I feel brave enough to lift some boards...

I understand the idea of a loose jumper in the stop tap too, is there a way of testing for this?? I ask as I assume replacement means turning off at the street and I'm not sure I'd be brave enough. Would get it done though if it cured the noise.

We've lived with the noise for some time to be honest, I was just amazed at the noise having completely disappeared today when the washing machine was on. I then realised I'd left the hose reel attached and turned on (although no water was flowing through the hose if you see what I mean). I just wondered why this hose reel had "cured" the problem. I'd leave the hose connected but I'm uncertain about the hoselock fittings, think it would be prudent to turn off as I usually do.

I see that the "shock arrester" perhaps cures a symptom, but doesn't address the underlying disease...

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