if you are talking about your plate heat exchanger you will need a descal machine. first turn off mains water supply to the boiler. secoundly cut the mains water feed to the boiler. then cut the hot water supply from the boiler to the taps. then connect the descal machine to the hot and cold pipes from the boiler start the descal machine this is pushing the chemicals through the heat exchanger. the machine will have a reverse switch on it wich will reverse the flow through exchanger once you fill there is good flow through boiler disconect machine re-fit pipe work and turn on the water mains again. turn on hot water tap and test the water at first will run all sorts of colours and have a slight smell but will clear. good luck hope this has helped . ps sorry about bad spelling
Dale, November 2007
