Replacing mercury thermostat with wifi. I have just a red and white wire connected to current thermostat and boiler. There is a green present, but it isn't hooked to anything on boiler. So I can use the green if I can identify the common on the boiler. I think I've found the blue common, but would like to confirm. I believe I just need to connect the green wire to the wifi thermostat and to the bottom post of the first picture, Green would then be my common. Am I on the right track?
My thermostat wiring is routed through several pumps, but final connection is here on the boiler.
Inside, a blue wire is connected to the bottom terminal of above picture (circled in blue on left), but there is also a red wire connected to that post. Circled on the right side is the blue wire connecting to post numbered 6.
Best Answer
Tap the bundle of yellow wires connected to the transformer
If you'll look at the wiring diagram for your system, you'll notice that your R terminal is connected via a red wire to the transformer via an inline fuse, and the other secondary terminal has yellow wires connected to it, as well as a green ground wire. Furthermore, there is a crimped splice of several yellow wires hanging below the transformer in your photo -- cutting that splice off, stripping the ends, and splicing a jumper to it using a wirenut (I'd use 14AWG yellow stranded THHN for this, it's cheap and easy enough to get) will let you bring that common wire out via a jumper to where it needs to be.