3 Port Valve Sticking??

[steved101]

Can a 3 port valve stick in the HTG only position? And if so, will it bring on the heating (by firing boiler/starting pump) constantly or ONLY when the programmer gives it power via the stat?

I know 2 port valves can stick and override the prorammer and room/cylinder stat as they have a permanent live feed going into them, but not sure about the 3 port mid?

 

[MANDATE]

There is a 'spring' that unwinds the motor, when power has been removed so it will return to HW position. Unless of course the valve spindle is seized or the spring is broken.
However! When in the CH only mode the boiler can only be fired up by the power output of the valves 'orange' wire.
The original source to feed this 'orange' wire comes from the 'white' wire and this comes from the room stat.