Another "don't have a C wire for thermostat" question. Been following along, and if I could identify the C connection at the furnace, I'd be all set. Unfortunately, it's not simply marked C or common. I have attached an image of the wiring diagram of my Utica MGB-2490 boiler. Am I correct in thinking that the blue wire terminating at the " PV 3" is what I'm looking for? thermostat only had 2 wires connected to R and W (extremely old Robert Shaw mercury type), there is a 3rd wire tucked in wall, I followed bundle to basement, it is not connected to anything, could use for C if I am sure what to connect it to. Any and all responses are appreciated
You want pin 5 instead
The connection you want is pin 5 of the connector you have marked, labeled "24V GND" on the diagram -- it has a black wire running form it. You can also tap the low-voltage junction between the white wire from the transformer and the black wires -- it's the same thing.