Just been out to a back boiler which has gravity circs and the hot water is ok with the heating on but the rads dont get hot and if they turn the heating off for h/water only it starts clanking and banging now the pump is newly fitted by the customer and its all vented,water in the tank so what shall i do next ive told them i will powerflush it but its not certain to cure it,im thinking of a heat exchanger sludged up,its a strange one and im the 4th person been out to it and she didnt tell me until id told her about it being sludged up that 2 others said that and one said scrap it get a new boiler which i thought was sound advice but she wants me to flush it and id like to know am i missing something that i should check before i go ahead cheers