Working and bled Heating now noisey

[hotunderthecollar11]

Wondered if someone can help!
I have a central heating system with a header tank which I recently drained, applied cleanser and refilled. The radiators have been bled and the system works fine - problem - boiler is knocking, banging spitting! I have re-bled the radiators but no further air is coming out.
What may be the cause of the banging? Is there rubbish that may have found it's way into the heat exchanger when the header refilled the system? Is it some air in the boiler that won't shift? Apart from this, the system appears to work fine.
Further info - the temp difference between the boiler in and out pipe is 3 degrees C

Cheers and thanks

 

[Fluffster]

What is the model and make of the boiler?
I would have a guess at an old cast iron unit such as Potterton netaheat.

Does sound like you have a flow obstruction into the heat exchanger.

Pete

 

[hotunderthecollar11]

Fluffster,

Thanks for the reply.
I have drained the system a second time with the pump isolated such that the head of water in the expansion tank drained through the boiler - I also set the three way valve to 'flushing' as ths may have been the cause of an airlock on refilling (with the pump isolator opened up for refillling). This seems to have cured the fault and made my wife happy!

Cheers,

Mark