Bang from hot water cylinder when heating stops

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Hello, I'm getting a single, loud bang and physical shudder in/from the hot water cylinder (indirect) when the heat cycle stops. The CH and HW are working ok but get a terrific clonk and shake and it's definitely from the cylinder.
What could that be?
Thanks a lot!
 
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Probably a 2 port zone valve closing.
Sometimes they just start doing it, even when they've been silent for years.
Sometimes it's because it's because the pump's still running when the valve closes ( no bypass?).
Sometimes it's because the valve is fitted backwards.
Air trapped in the pipes can do odd things - see if there's a bleed point.
 
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Thanks for that. There is a bleed point just above the cylinder coil inlet and I checked that it is bled, if that's the one you mean?
Is it likely that replacing the zone valve (downstairs) would stop the bang up in the cylinder?
 
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I've had some correspondence with Honeywell over this; our 3 way valve was banging when the CH was going off. I believed that the clonking was caused by the hard rubber ball inside the valve getting worn or crudded up or the pivot wearing so I drained down and replaced it with a new valve, which cured the problem for a few months. When it started again I sent the old valve back to Honeywell with a letter of complaint. They asked me to send them a picture of the installation, which I did, and duly received a lengthy "get out" letter bullshitting about water pressure and pointing out that the valve was installed with curves in the pipework too close, so it was my fault.

They achieved their objective because I can't be bothered to argue with them any more and have given up and we just live with the occasional loud clonk.
 
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