Ok reckon I've got this now. In order for the CH to heat up properly, there has to be power on grey wire. Achieved by power from one of the two blue wires, either because HW isn't on or HW is satisfied.
This definitely happens when I turn off the HW, and also when I turn the stat right down. My thought is maybe with the stat at its normal operating setting, it is not reaching a satisfied state, and so not putting power to the grey wire. This would point to (as newgas said) a faulty cylinder stat?
I will do some more tests tonight, and check the pipe temperature either side of the valve as you suggest, to confirm the valve is moving. I think its ok though, as I can get CH on its own.
Thanks for sticking with me - really appreciate it.
This definitely happens when I turn off the HW, and also when I turn the stat right down. My thought is maybe with the stat at its normal operating setting, it is not reaching a satisfied state, and so not putting power to the grey wire. This would point to (as newgas said) a faulty cylinder stat?
I will do some more tests tonight, and check the pipe temperature either side of the valve as you suggest, to confirm the valve is moving. I think its ok though, as I can get CH on its own.
Thanks for sticking with me - really appreciate it.