My boiler is playing up again, so it looks like the heat exchanger will have to come out tomorrow for its periodic clean up. Last time I did it, there was a lot of 'blackness' on the internal walls of the exchanger, but no readily seen build up of sludge/muck in it. I gave it a blast through with a hose pipe, and it worked fine again that time, but it has only been 4 months since I last did it.
So, the query - is there anything I can pur into the exchanger to give it a good clean internally? IIRC, it is an aluminium casting. Thanks Alan.