Odd one here...
I have an indirect, pumped central heating system using a Siemens RWB9 controller. The heating works, the hot water works, the diverter valve seems to work.
If the water cylinder is full of hot water, the central heating will not come on. through the timer or directly from the controller. When I run the hot water / empty the cylinder partway, the Central heat comes on.
It started when an addition was put on the house. I've traced all the pipes, done everything short of changing the controller. Not sure what else to check
Thanks