I recently bought a Sensi Thermostat with Wifi to replace my vintage one. I thought it would work with my 2-wire heat only system because it has batteries , but found out that using the Wifi feature with batteries only with drain them in only a couple days. After searching forums and looking at the Sensi website, I will need to add a C wire.
I read that I can repurpose the fan wire (G) as a C wire by connecting it to the C terminal in the circuit board. I looked at the thermostat wire and found the fan wire (G) unused and stuffed into the wall. However, my system is so old that it doesn't have a circuit board, only a mess of wire nuts.
Where I should connect the C wire to on my old system?