Banging water pipes - radiators


Hi - I have a pressurised domestic boiler system that seemed to be working fine before I had a house extension and several radiators added. The diverter valve failed once and was replaced and after this time, the pipes bang quite loudly when the thermostat switches in to stop the boiler. The banging only takes place when the radiators are on and seems to be in the area to the top of the boiler / divertor valve / pipes into the floorboards- there is no noise caused in Summer when I only have the hot water side working,a nd I have secured the pipes near to the divertor valve with brackets to the floor joists (to no avail).
The system seems to keep pressure well (it can be topped up from the rising main), but I periodically (perhaps once every month) have to bleed air from a few radiators (so air does seem to be getting in somehow).
Does anyone have any idea what could be causing this? I have had two plumbers in who were clearly lacking in ideas- one replaced the divertor valve, and another put a larger pressure overflow vessel in the system - both have made no difference!
Help Thanks Simon
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My system is a traditional gravity filled pumped system so this might be b*ll*cks!, but I had a similar problem that was solved by turning the water temperature on the boiler down a bit. I assume that when the pump switched off the water in the boiler overheated. Probably need a stat but sorted for now.
Try turning yours down to lowest and see if the problem goes away and start from there!
HTH. Bill.

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Bill - thanks - I will give that a go and see if it does the trick! Regards Simon
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