Thanks for your persistence. I did already remove the actuator and move the valve spindle. Yep, it's only about 20 degrees of rotation, I'd say it was easy to move just with fingers.
Also the valve itself is working - I can change it from A, to both, to B as witnessed by the temperature of the outflow pipes.
Considering the 'sticky' theory - that would seem to agree with what I've seen, where a gentle push from mid-position on the manual over-ride gets it started, then it goes to the end position by itself. But maybe this is in the actuator, not the valve. The internal circuit of these MVs applies rectified DC to the motor coils, which could cause permanent magnetism.
Some dull history that might explain this further:
Another factor here is that a couple of months ago I changed the tank stat. I simply refitted to the existing cable and remade all connections at the tank end, leaving the wiring centre alone. Prior to that, we had CH + HW but the CH never felt properly hot. The stat was broken internally, with the bimetal strip flapping around loose. Complete crap. I think it's been that way ever since the original install when the house was built. With a replacement tank stat, the CH gets MUCH hotter. Now some other things make sense. With a broken tank stat (2 months ago)
- Boiler was running much more than it needed to
- Mid-position valve would have never gone to end-position, as cyl would never be SAT
- This means grey-wire-related faults (if there are any) would never be seen (so that's the other theory)
- This also means the MV would be sitting permanently in mid-position, with half-wave-rectified DC going through the motor permanently
I'm pretty sure it's not a wiring fault, but will check further. In the meantime, perhaps that could have magnetised the motor enough to cause it to 'stick'.
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