you can remove the heat exchanger by turning of your main cold water supply open your hot tap to drain your dhw unscrew the exchanger which is staight forward. then soak in descaler overnight. u can buy the descaler which we use for powerflushing from plumbcentre etc.you can't damage the exchanger taking it out it just pulls off once unsrewed.
ITS NOTHING TO DO WITH YOUR PUMP
AND CHEAP THAN NEW ONE
HI-SPEC PLUMBING., July 2006