The route of the "please fire up" signal into the boiler is from the timer programmer - to the room thermostat - to the Mid Position Valve on the White wire and from there to the boiler on the Orange wire.
There is conditioning for this signal inside the boiler itself (air pressure switch and overheat stat) but these are OK for Hot Water and may be discounted.
It would be good to have somebody observe / listen to the MPV as you select C/H to come on - it should drive fully over and make a microswitch to send the fire up signal out.
The problem you have is very likely to be caused by either the Timer/Programmer - the Room Thermostat - the Mid Position Valve - very probably the latter.
I have no qualifications or experience with respect to boilers - my advice is next to worthless.
Good luck...
Peccavi, March 2010
