Central Heating Bang when it Switches Off

8 Jan 2012
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United Kingdom

I was hoping someone could help or offer any suggestions:

I have a megaflow and a gas boiler with 3 X 2 port motorised valves in the loft along with the megaflow.

1 valve is for the underfloor heating which is fine, the second is for the hot water which is fine but I have a problem with the central heating.
When the central heating is only on and it switches off there is a loud bang in the loft. I assumed it was the motorised valve but I when I checked it doesn't appear to be the motorised value making the bang sound.

However if the central heating is on with either the water or underfloor heating and the central heating stops it doesn't make a banging sound and everything is ok?

Any ideas as to why when only the central heating is on and shuts it would make a loud sound in the loft.

The boiler is in the kitchen and is fine the noise is definitely in the loft but I can't seem to locate it?

Thanks in advance
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I've checked and its the right way.

The problem only started a few weeks ago it was fine before

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