this boiler overheating within 20 minutes, although the radiators become hot. In this case, hot water and central heating will never come again, although the radiators become cold. I have to reset, the central heating will work, but overheat will come again within 20 minutes. error code is 7 which is FLUE GAS OVERHEAT. I think there is something wrong. Could you please give me some advices? thanks