Probably furred up if its never been cleaned out.
Another thing is a lose plate my Potterton rattled and it was a lose cover plate. Kettling is usally caused by furred up/ furring up of the water jacket. If you have to have a new boiler fitted then it will be a condensing one. Make sure the system is estimated to have a power cleanse/flush as all boiler manufacturers insist on it or warranty void.
This must be done BEFORE the new boiler is installed. Apart from the higher price of combie boilers and having to have the pipework changed I stuck to the conventional and a condensing boiler is my replacement. With airing cupboard and hot water storage tank if it has an immersion in the top (black/white cap) you have back up hot water heater if the boiler goes wrong.
bob uk
November 2013
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