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. current thermostat connection to 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. Blue wire on inside connects to a post that appears to be number 6 on the right side-circled

Wiring diagram Blue wire circled.


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.

  • Thanks, I get it. The red coming out of the transformer is the hot and the yellow is the common. The sure fire way to get a common is to just connect it to the yellow. Easiest spot is at the wire nut/crimped splice. Thanks again. – Jim Oct 17 '18 at 16:59
  • 1
    @Jim -- we thank people around here by upvoting and/or accepting their answers :) – ThreePhaseEel Oct 17 '18 at 22:40

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