rads warm up when hot water turned on vokera unica 36 he?

That will be because the heated water is being shared between the radiators and the hot water storage tank when it should not be being shared. Consider that the diverter valve is stuck in its mid position. You can override it manually.

I just guess - I have no experience or qualification here.
The heat exchanger is not involved in where the hot water is sent to.

Good Luck...
October 2016