Have added an underfloor zone to existing heating have issue with temperatures

[Timlazisa]

So I have just added an underfloor zone to a house extension. Boiler is Worcester Greenstar 30Cdi. has a Comfort II RF stat fitted that controls the radiator zone. I have added a zone valve to my new zone and used the 230V side of the control board to fire the boiler. the boiler fires fine but will not rise above 20 C. However, if I engage the Comfort II the boiler reaches normal operating temp. My question is do I need to bin the Comfort II or is there something else I can change to make it work normally on the underfloor zone alone?

thanks in advance

Tim