Hello, I'm having an odd issue with my mid-position valve. This is a bit lengthy as I want explain the steps I've taken... I have a y-plan heating system, with a 3-way mid-position valve next to my HW cylinder. My hot water is working fine, and when there is both water and heating demand, the radiators get warm. If there is only heating demand, the mid-position actuator makes a clicking sound, you can see it trying (and failing) to move to the heating only position, and the primary pump does not start. I've made a video of this if you want to see (the volume needs to be pretty hear the clicking): I replaced the actuator, and the first time it went all the way to heating (and the pump started), then on the second attempt got stuck in the mid-position again, with the same clicking. I had a plumber round and he replaced the valve body. Same issue. I thought I'd try draining the system, filling with cleaner and replacing the actuator head again, just in case it have been damaged whilst on the old valve. I did that, but this time I did not put the (new) actuator back on the valve, started the system on heating only. The unmounted actuator moved all the way across to the H position. All good. I add HW by turning it on at the programmer and adjusting the cyl. stat, and the actuator moves to the M position. Also good. Now, if I turn off the HW (by reducing the stat setting on the cyl.), presto: The actuator get's 'stuck' in that clicking mode trying to move to H. This actuator has never been on the body!! Even more interestingly. If I start it CH only, the actuator moves across. If I then turn off CH demand from the stat, the boiler keeps running and the head stays in H. This seems electrics related? Could the control unit (Sunvic SP50) be the issue? I'm not really sure where to go next... any advice would be gratefully received.