I have a Potterton Suprima 50 and a Boilermate II, both are 8yrs old nearly. The boiler has now developed a knock, presumably due to a build up of scale. Several products I have looked at using to descale the boiler/system specify that it should not be used on primatic sytems, ie where the mains pressure is used for hot water, as in Boilermate II. Does anyone have a solution, other than replacement of the boiler or heat exchanger? Thanks