Overheating - Cold Water Tank Boiling

Hi,

We have a PulsaCoil A Class boiler.
It is not set up on twin supply (economy 7).

Several times now the error light has flashed quickly and the cold water tank above the boiler unit heats to the point where it boils.

Any suggestions on why this is happening and how it can be fixed please?


Many Thanks,
Matthew

Most likely relays are faulty.
There is a mod kit for the PC A class.
Not a DIY job though.
Mike