How to add C wire to a very old 2-wire Lennox G16 Gas Furnace?

Question

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?

Answer 1

According to the wiring diagram for your system (i.e. Figure 2 in the linked PDF) -- the yellow wire on the 24VAC side is the C wire (it's the wire that goes to the compressor but not to the 'stat in the diagram). Just incorporate the green wire from the thermostat cable into the nut with the yellow and green wires in it on the 24VAC side, and you'll be set!