we have a open vented ten rad ten year old system run by a worcester greenstar 24ri. we replaced a failed pump to a system that was only managing a max of 4 rads luke warm. we have replaced 3 way switch and actuvator then we discovered the system was critically sludged up so removed and flushed all rads and put cleaner in the system later draining and flushing then repeating the process a few weeks later when once more the radiators went luke warm. all seemed well and all rads hot and once more rads went cold. The flushed clean water this time had quite a lot of metalic grit residue. The boiler is now locking out very quickly with a flashing blue light. There is no air in the sytem and we have vented the heat exchanger. could the heat exchanger still be blocked after several cleaning applications? or can they fail altogether or could this be another fault? any ideas?