40 year old indirect cylinder coil recently ruptured and contaminated the heating system. 2 power flushes have cleaned pipework and rads but not the heat exchanger. Have used x800 twice and got lots of black magnetite out and some particles of rust, but boiler starts kettling as it approaches 70oC. The boiler is a Gloworm 12hxi open system design. Tried anti-kettling additives to no avail. Any help appreciated. Many thanks
