3-way valve problem?

[Slaydeuk]

I'm now swamped with information - and thanks to everyone for that. At risk of boring everyone to death, I'll drag this out a little further.

Ive not really carried out much checking as I've been waiting for the fault to recur. I've had the valve actuator off and removed the motor. The valve moves smoothly and returns to the mid-position. That's it!

As regards the status of the system when the fault occurs, the only thing I know for certain is that it has happened when the controller is showing lights for both CH and HW. I suspect that, as the recent case happened late in the evening, the cylinder stat was satisfied at that time. This would mean that the boiler is getting its power via a micro switch in the valve head. If this switch doesn't make, the boiler is effectively switched off until the cylinder stat calls for heat. If, before this happens, I realise I am getting cold, switch off the system, causing the valve to return to the mid position. I then switch on again and the motor moves over and this time the micro switch makes - hey presto! the boiler fires up and all is well. How does that sound for a scenario? Maybe I should replace that micro switch?

In any case, I intend to do some lose connection checking around the system, as has been suggested.

 

[MANDATE]

Can I just comment something you quoted.
'switch off the system, causing the valve to return to the mid position'.
If it does what you say, then the valve is at fault, but you may be mistaken regarding its position
Switching off the system and valve should return to HW position.
The valve has 3 positions. Without any power the valve should be at the HW position. When HW is called for the valve does not move.

It is only when CH is called for that power is supplied to drive the motor forward. power is applied to white wire for the first half of range, then power is applied to the grey wire for last half of range.

The motor drives a quadrant which can not go beyond CH position. The quadrant is 'spring loaded' so if the power that is holding the valve is switched off the return spring unwinds the quadrant/motor back to the rest position which is the HW position.

 

[Slaydeuk]

Thanks for putting me straight on that one. However, I think it still leaves my hypothesis as to a possible cause of the problem valid. As I switch off the power, the valve moves to the HW position (not the mid as I said) and then moves back to the CH position as I switch the power back on again, and the switch works this time and the boiler fires up. This assumes HW satisfied and CH calling, a likely scenario at the time of the failures.

This seems to me the most likely explanation for why switching off and on resets the system.