Raven heat CSI AAA T 780 series high efficiency condensing boiler?

There will be a valve that directs heated water to radiators or the hot water tank - one or the other and sometimes to both at the same time. My guess is that your diverter valve is stuck and will not return to the hot water only position.

https://www.youtube.com/watch?v=T7UTDtrmcvM

https://www.youtube.com/watch?v=M9DkNwVL4Dc

You can take the motorised head off and operate the sticky valve with a spanner or pliers.

Good Luck...
March 2017