clean heat exchanger vaillant ecotec 831?
 
                hello,
can anyone give me a idiots guide to removing, cleaning and replacing a valliant ecotrec 831 heat exchanger? 
we've got intermittant hot water due to the scale build up in the exchanger (my plumber mate cleaned it a couple of moths ago, but has gone on holiday.typical!), so i'm going to desludge the system and clean again.
Ta!
            
             edwardo
            
 November 2012