you can remove actuator head on divertor valve without draining down boiler & check the pin moves in & out for CH & DHW
if it doesnt actuator faulty
if it does then external system fault
are there any isolation valves on the CH pipes [apart from boiler valves]
something has to be stopping the water circulating around the system
if not even 1 rad is heating up then the problem must be between boiler & 1st rad
manwithnoname
April 2012