No Hot water but rads come on

[PhillleC]

Hi a fault has developed in my central heating system.

I don't seem to have any hot water when the system is on hot water only but it seem to heat the radiators. there is also an electric emersion heater in the tank which still heats the water up. i have a baxi back boiler i have checked the header tank that seems to be ok could it be a faulty controller unit? when i have the central heating on that is working ok

 

[Diyisfun]

Im not a plumber, but it may be the diverter valve (2/3 port thing).
If you know where it is, it can be moved manually to check it.