Hi Craig will try to be of help without being too technical its easy for us pros but hard for a DIYer to understand, OK both valves are identical, when there is a demand for CH or HW the required vale gets a power supply to a motor which opens the required valve, once this valve is fully open then the valve makes a small switch that in turn asks the boiler to fire and the pump to run, when the demand stops then the motor is de-energised and the motor spring closes the valve, in normal circumstances then the micro switch will also break the supply to the boiler and the pump , however these switches are prone to sticking and causing a false demand, both valves have the same switches and bot have a grey wire and an orange wire, they are simply switches and if one is sticking it will give the problem that you have .
