Hi guys - done my cutting and pasting (finally got drawings in correct order, with correct blurb - thanks). However, in trying to identify my client's problem, the sequence is somewhat reversed. Let me explain:
If he starts with CH only on, then it seems the valve will be fully diverted to Port A open due to the 240V on the grey wire (from DHW off). The 240V from the room stat will go via the microswitch(es) on the valve to the orange wire and hence power the boiler/pump.
If the timer now switches on the DHW then the 240V on the grey wire disappears, the valve starts returning under spring force and is stopped at the mid position due to the first microswitch and 240V on the white wire from the room stat going to the diode then the motor (interesting stuff about the DC braking effect on an AC synchronous motor in linked post!).
So there we are at the illustration from Chrishutt on the previous page, both CH and DHW calling for heat and the valve held mid position. But now suppose the room stat is satisfied (cylinder still calling) and the 240v on the white wire disappears. All voltage to the motor will also disappear, causing the ball to return to it's default Port B DHW only open position (thus it seems that the valve hold function only applies to the cycling of the cylinder stat - am I right?).
Now this is exactly where the problem kicks in: the room stat 'on' indicator stays a-glow - even when the dial is turned down manually - and the CH goes up & up uncontrollably (i.e. the house gets too hot). This must mean that the valve is still open to Port A, despite the room stat being satisfied, and the 'on' indicator would indeed suggest voltage still on the white wire, and therefore the diode circuit.
This is beginning to sound like a fault with the room stat, though this has been changed since my client inherited this problem when he bought the house.
Or perhaps the valve is sticking in mid-position....now that I think about it, whenever I moved the power head lever from auto to manual, I don't recall feeling any resistance for the first half of the lever's travel. Being new to this, I never gave it any thought, but now that may indeed indicate a stuck ball. Sound feasable?
I'm going back tomorrow to work on it. The stuck ball idea makes some sense: is this a common thing to happen? If so, is it likely to require a change of power head or valve or both? Thanks again guys, I've learned a lot of good stuff here and may be close to cracking it!