Water Hammer Arrestor

[F]

After listening to water hammer on the pipes for the last 30 years I've finally got round to doing something about it (best not to rush these things) and fitted an arrestor -

've had most of the plumbing in the house exposed at some time or other but have never been able to find the source of the problem. Only a couple of 3M or 4M stretches have never been able to be checked so I fitted it in a section as close to what I presumed to be the source of the hammer as possible.

It's not worked. Nothing has changed.

Do the arrestors work (if they're going to) as soon as they are fitted?

The top of it has what looks like a bicycle pump connector. Do I need to get some pressure into it?

TIA

[Andrew Gabriel]

I don't know if they come pressurised (my boiler expansion vessel did). If you pressurise it to half your water pressure, the diaphragm will be end up midway when you connect up to the supply, so it will take both +ve and -ve pressure shocks.

[fred]

In article , F writes

I agree it's best not to rush these things.

I assume by water hammer you mean banging in the pipes when a tap is turned off sharply?

It's handy to understand that this kind of water hammer is about shock waves travelling along a pipe when a flow is interrupted and making a bang when the shock hits a restriction in the pipework, whether that be the incoming stop-c*ck, a narrowing in pipework or if really unlucky, a side limb of a T joint.

That said, it's common for water hammer to be solved by simply fixing down the pipes in a system securely so that they can't jump about when a pressure wave is passing through them. That is certainly the case when it is a vibration in pipes when the pipes are on rather than a bang when they are turned off. Try this first.

The device you linked to is a typical one and they are ready to use out of the box so no need to pump them up, and as they have such a low volume you could easily empty them by just trying to measure the pressure so best left alone.

They can be a bit hit or miss but they are generally best placed to catch the shock wave at the opposite end of the pipework from the source as that is where the sharpest shock is. If you've placed it at the end of a branch then I'm afraid it's probably the wrong place. I would first place one of these as close as possible to the incoming stop-c*ck.

See also:

[Lobster]

I fitted one of these recently to cure a problem with a noisy toilet cistern; I fitted it at a convenient location in the cold water pipework (several meters away from the loo) but it did absolutely nothing; I then demounted it and repositioned it immediately below the cistern where it's unsightly but reasonably effective. Certainly not perfect though.

David

[narsanam]

..

[F]

I was wrong. It's not worked but something has changed: it's louder!

As it's louder, I seem to have got the correct section of pipe.

I'll certainly be taking it out but there's no-where else on that section to put it as the rest of it is either boxed in or under a tiled floor where I fixed the pipes to joists before tiling.

[Andrew Gabriel]

One possible cause of water hammer is a deadleg with air trapped in it. Now, a Water Hammer Arrestor is simulating a deadleg with air trapped in it, so you might want to see if you have some more real ones on the pipework.

[John]

I believe that cheap washing machine hoses can lead to water hammer as they are slightly elastic.

[Chris Hogg]

It is sometimes recommended to turn down the main stopcock slightly to decrease the flow of water in the first place when a tap is opened, so that when the tap is turned off suddenly there's less of a shock wave. This didn't work for me, but, and counter-intuitively, opening the stopcock up full did stop the hammer. I have no explanation.

Something else I've seen suggested is a flat baffle plate fixed horizontally under the float on the ball valve in the cold water tank. This helps stop the float from 'bouncing' and hence perpetuating the knocking when a tap is turned off suddenly. Years ago I lashed up something along those lines from a piece of aluminium plate about 30cm square, held in place with copper wire stripped from mains cable. Can't remember if it had any effect though!

[F]

Thanks for the suggestion but, some time ago, I went up into the loft and held the floats up to stop either of the valves from opening. Management operated the cold water tap and the hammer was still there.

[F]

I'm pretty sure there's a horizontal dead-leg (remnant of a water supply into the garage) and I have long suspected it's the problem but I can't get into that space to check.

Seems odd that introducing a simulated dead-leg is supposed to cure the problem...

[Chris K]

Managed to stop this with a home made version of the hammer arrestor (22mm pipe with cycle valve at the top) but it needed regular top ups of air with a cycle pump. Finally stopped for good when I replaced the toilet and used a flexible coupler. A 500mm one arranged in a loop seemed quite effective as a cheap hammer arrestor.

Chris K

[Andy Champ]

Changes the resonant frequency of the system.

Andy

[F]

I've got five of these on the cold pipes in one place or another but the hammer hasn't changed since I installed them.

There's a few of us on the street who would like to meet the 'plumber' who was employed when they built the houses. Pipes not fixed and left to hang are quite common: a neighbour had one that was left resting on a nail until years of movement wore a hole through it. I've tried to rectify things as I've worked my way through extending and refurbishing (now refurbishing the refurbishments!) the house over the past 30 years but there are a couple of areas that just are not easily accessible.