Can anyone help, had problem earlier this year which seemed to be sorted. Just switched central heating back on and immediately the hot water went cold. It is intermittent doesn't matter if central heating is on or off. It calls for heat, 5 to 10 seconds later shuts down, similar time lapse and cuts in again and repeats this cycle. The water starts hot goes cool and then hot again. The plumber changed the heat exchanger and flushed through the system took all day so hopefully was done correctly. He also fitted a BoilerMag as we are a hard water area. Any suggestions