I've a Baxi Bermuda back boiler that's banging and crashing away. I think it's what's called 'kettling', Presumably caused by the build-up of sludge and lime scale. It's just over ten years old.
It's a thermo-syphon hot water system with pumped water to the radiators. I've a t'stat on the hot water cylinder to call for heat when required and a room 'stat controlling the pump and boiler. Not perfect I know and the wiring's a bit of a nightmare (self designed with Venn diagrams and impressed an electrician who later showed me a near identical circuit in a book).
It replaced a Majorca Gloworm back boiler that was in the place when we moved in '75, that displayed the same symptoms and upon inspection had a cracked (cast iron) heat exchanger. The original system had a 'semi-primatic' (is that the right term?) hot water system that was changed out before the boiler was replaced around ten years ago. That meant the original system couldn't have corrosion inhibitors added until the hot water cylinder was replaced. All the radiators were removed and flushed through when the system was changed from semi-primatic and a header tank fitted. Corrosion inhibitor was also added.
I intend to empty the header tank and remove the sludge I can see in the bottom. I'll refill it, close off all the radiators, open them up one at a time and flush them out. I'll do the same with the boiler.
Because the boiler's at the bottom of its own 'loop', with its own drain, I can use a hose to fill just the boiler and I've got an old container of Fernox DS3 which I intend to use to de-scale just the boiler and then thoroughly flush.
Any comments or advice from the plumbing specialists here before I proceed (I don't intend going into the loft until this warm spell's over), particularly for additives afterwards.
I'm trying to put off the day I'll have to fork out for a new condensing boiler (probably in the loft) for as long as possible.
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CH boiler maintenance 
CH boiler maintenance 