I have a Gloworm Betacom boiler on Gloworm (Homeserve) extended warranty. Boiler is 1.5 years old. All pipes and radiators were changed 8 months ago. All was fine for 6 months and than hot water started to change from hot to cold every 20-30 seconds when you run hot water. Boiler man changed heat exchanger which was blocked. All started to work but failed same way in 2-3 weeks. Same boiler man came around, took the heat exchanger away and showed me black flakes coming out of heating side of the heat exchanger. He said that they could not change it again as looks like my heating system is dirty. We changed water in system, used all required chemicals, installed magnetic filter before the boiler on the heating system, after that they (Gloworm) have changed heat exchanger and the pump again and all was working fine for another 4 weeks. Than the same story again. Whole system is brand new, magnetic filter installed, water was changed and all required chemicals used. Why is that failing again? What can be the source of that black stuff which blocks it? Any ideas? What's next to do?