Water hammer?

[disclaimer: non-diy person, me]
I've recently moved into a mid-70s two-storey detached house. The house has a new Worcester Greenstar C/H system, installed just before we moved in.
The problem is that first thing in the morning when the hot tap is opened there is a LOUD noise like a hammer-drill from upstairs, lasting for up to a minute. This only happens if the tap hasn't been opened for several hours. If the tap is closed while this is going on, the noise takes a couple of seconds to fade away. I can feel a couple of the pipes in the airing cupboard vibrating strongly, particularly a flexible hose that goes to what seems to be some sort of pressure vessel above the H/W cylinder.
The installer has been back and said something about air in the system causing this problem. He reckoned that he had fixed it by doing something in the loft (ball valve?) but the noise returned after a couple of days. Before I call him back again can someone please give me a clue as to what might be happening? Just to guard against bullsh^W flannel, as it were.
--
^Ï^. Sn!pe <http://snipeco.ath.cx/~snipe/

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On 15 Feb, snipped-for-privacy@spambin.fsnet.co.uk (Sn!pe) wrote:

Wotcher doing out of the shed, tea room or pub?

I've had similar troubles with a conventional hot water system and a combination of (hot) tap openings (and rapid closure) and bog flushes. This was alleviated to some extent by introducing air /into/ the system to damp resonances. On one occasion reversing a washer in the bog cystern cured it for a while (moved the resonant frequency).
I suspect your water pressure is higher overnight and there is a non return valve on the hot water system, thus holding teh higher pressure. The release of this overpressure when you open the hot tap is setting up a resonance in the pipes (AKA water hammer). A loose pipe may cause it. De-tuning the resonance will cure it. Finding what is resonating can be the problem. In my case it was started by flushing the bog followed by opening the loft tank ball valve by running hot, and almost simultaniously opening and shutting rapidly the cold tap, thus setting up two ball valves oscilating in resonance. One of my offspring could tune this up to a grand finale. No-one else could.
In your case you have a spring in the form of a hose, and another as an air receiver.
Reducing teh Q of the tuned circuits, by adding damping, or altering the resonant frequencies/mutual coupling should cure the problem.
I leave this as an exercise in applied mathematics and lateral RF engineering.
Mine's one of the black stuff!
--
B Thumbs
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Mornin' Mr Thumbs. I'm here as an earnest seeker after truth and enlightenment into the mysteries of plumbing, of course. Much as I love the shed and so forth I find that one is more likely to get a sensible answer in a technical group...
[...]

Ta for all that, I hadn't thought of it in terms of Q before. Thinking about it, the solution proposed by RobertL of fitting a shock arrester is vaguely analogous to connecting a decoupling capacitor to an HT rail.
--
^Ï^. Sn!pe <http://snipeco.ath.cx/~snipe/

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On Feb 15, 10:38 am, snipped-for-privacy@spambin.fsnet.co.uk (Sn!pe) wrote:

Sounds exactly like waterhammer to me. You can fit little expansion vessels that are specially designed to damp out this kindd of thing.
For example: the "shock arrester" about 1/3 way down this page. http://www.bes.co.uk/products/109.asp
Robert
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[...

Thanks, Robert, very interesting. Now I cast my mind back I think I've heard of these before. I'll ask the installer what he thinks when I fetch him back.
--
^Ï^. Sn!pe <http://snipeco.ath.cx/~snipe/

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I fitted one last month, the pipes were regularly banging here when next door had a washing machine or something similar on. Made my mains inlet bang throughout the house. I fitted this just above the stop cock: <http://www.toolstation.com/shop/Plumbing/Valves/Potable+Water+Shock+Arr estor+12+MBSP/d20/sd2696/p93058>
Not had any hammer since. It has an adjustable valve in it, pump it up with a bike pump. Unfortunately, no instructions with it, but it seems to work well as supplied. Alan.
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On 17/02/2010 09:01 RobertL wrote:

Unfortunately, not guaranteed to work. I installed one and the hammer is still there...
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F



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Interesting, maybe it's in the wrong place. It's not always easy to work out where the hammer is originating from.
Robert
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On Feb 15, 10:38 am, snipped-for-privacy@spambin.fsnet.co.uk (Sn!pe) wrote:

http://wiki.diyfaq.org.uk/index.php?title=Noisy_Pipes
NT
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Sn!pe wrote:

It can be the tap washer bouncing up and down, very common if the washer is one of those half round rubber ones (domelike).
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