I've read several threads here and on the other websites but still can't figure out how to add C wire to my boiler wiring. I would like to replace my old mercury thermostat with Nest E or any other WiFi thermostats. My current one is connected with 2 wires (Red and White) but there is another green wire that is not connected to anything at the thermostat nor at the boiler. I am hoping I can convert that wire to "C" so I can avoid running additional wire.

First about my current set up: I only have a heat boiler (no HVAC yet) with regular radiators in rooms that receive hot water from the boiler. Wire that leads from boiler to thermostat consists or Red and White wires that are connected to thermostat and additional Green wire that is not connected on either end.

Manual to my boiler

Old thermostat wiring As you can see Red and White wires are connected to R and Y terminals on my old thermostat. If I connect them to Nest E boiler won't turn on and I hear constant clicking at the boiler side.

Boiler wiring Here is where it gets confusing. Other threads mention to wire cables R and C at the boiler and then connect them directly to thermostat. My cables however are now connected to TSTAT (white wire) and some black part above the boiler (Red wire). R and C wires from the board are only connected to black part above my boiler and not to the thermostat cables at all. Picture from the link explains this much better.

Similar Thread Seems like I have a same boiler but my wiring is currently configured differently as I've explained above.

Any feedback is appreciated. I am an IT guy but went through several construction jobs when I was in college so I am not afraid to do some work. However, if you guys think it is better to call specialist here let me know and I will do that. Problem is they don't seem to like these new thermostats as I was already told to keep old mercury one by couple of them when I was asking about how to make this work. Let me know if you need any additional information.

Thanks, Peter


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