Water hammer in central heating system pipes

Hi,
I have an open-vented, S-plan system with two 2-port zone valves, 28mm to hot water cylinder and 22 mm, dropping to 15mm, to radiators. There is an automatic bypass valve fitted in a loop-back circuit.
Last winter I started to get a banging noise from the central heating system. The noise was a load bang in the pipework that occured intermittently shortly after the heating cut out.
I think this is described as water-hammer?
As it happened, I was in need of a new boiler anyway, and arranged for British Gas to come and install a new boiler, pump and zone valves. (Pause to wait for traditional BG insults). The idea here was to change everything that could cause the water hammer.
The radiators in the system (13 mixed sizes) were newly fitted about 3 years ago.
So, BG went ahead gave the system a chemical clean, and installed a 300+ HE boiler, new pump and new bypass valve but, due to some admin cock-up, new zone valves were not fitted, so I'm left with the old ones.
When I complained about this, BG told me that there is nothing wrong with my existing zone valves and that the replacement bypass valve should fix the loud-bang-as-zone-valve-closes problem.
Sadly, this turned out not to be the case, and I am still getting the load bangs with the (mostly) all-new system.
Would any CH guru out there care to suggest what is causing the water hammer and what I need to get BG to do to fix it? Is it possible/likely that the zone valves are to blame or am I fixating on them unnecessarily?
I'm currently waiting a call-back from BG and would like to prepare myself with what options there are to diagnose and fix this problem.
Many thanks.
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wrote:

You may want to check the setting of the auto bypass valve. I had a boiler replaced last year, and it looked like the installer forgot to set up the bypass valve. This caused some banging when the stats switched. I adjusted the bypass and it cleared the problem.
John
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"JohnW" wrote
snip...

Seconded.
Although the poor setting of mine didn't initially cause banging - that came later. Because the by-pass valve never opened, the boiler went into alarm state when all the zone valves closed. Also the pump over-ran for ages to try to dissipate the boiler heat.
Banging started much later and was resolved (for now at least) by turning off the system, winding the by-pass valve fully closed - then fully open - then back to its correct setting.
Maybe it would be better to sling these things and just use the old fashioned 15mm balance line to ensure some flow is maintained across the boiler?
Phil
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Thanks for the idea's guys. The bypass valve (honeywell du144) is set to the widest open position. At least I think so. The brass knurled nut is fully anti-clockwise. FWIW I am also getting very long pump over-run times. The boiler is set to 75c.
Can we rule out a faulty zone valve as the cause? My theory is that the wet part of the valve is sometimes sticking when the motor is deactivated, but then the return spring wins the battle and closes the valve with a short sharp shock. Most of the advice I came across in the web for water hammer was to sort out the pipework but in this case the pipework has been working for the last 10 years (or so) and the water hammer only showed up ~6 months ago.
Thanks again....
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Mike Smith wrote:

I wonder if you might have an old directional zone valve that has been fittted the wrong way around? It may be that with a bit of slack developing in the actuating mechanism, the ball in the valve is bouncing on its seat?
Work out which way the water is flowing though your valves (feel for which side heats up first when the system is turned on) and make sure that this agrees with the direction indicated by the arrow (if present) on your motorised valve.
Of course, if it was agreed that BG were supposed to replace the valves anyway then get the idle buggers back to do the job they were paid for.
Tim
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<snip>

Tim, zone valve flow direction is correct. The system has been working fine for many years. Thanks for the suggestion though.
p.s. I got a refund from BG for the zone valve installation that wasn't carried out.
My worry is that BG is correct and my existing zone valves are just fine.
That leaves me wondering just what *has* happened with my system and how to fix it.
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