So some background, converting from an oil to a gas furnace and was looking at adding a smart thermostat. Two things that kind of raised questions and I cant seem to find much answers too. One is the thermostat, the original one only has a red and white which seems typical, what I couldnt find an answer to is that they seemed to be connected to 12/2. Where the thermostat location almost feels like the previous owner replaced a switch with the thermostat. Another question is, where do I add the common wire in the furnace? I tried following the old wire connected to it and it doesnt look like there is a spot for a common wire or it is called something else.

So I guess my two questions are:

  1. Why would the old thermostat wires be connected to 12/2?

  2. If possible where do I connect the common wire inside the furnace? Schematic on furnace door


  • A focused pic of the insides of the furnace, and one of what the thermostat is connected to would be most helpful. Please edit those in. – FreeMan Feb 16 at 11:49

On the L8148E terminal B2 is the common terminal, connect your C wire there.

Source: https://www.youtube.com/watch?v=BZJ2tch3_oc&t=200

  • Thank you for the response, I could not find that video but that is helpful. Do you know why the thermostat would be connected to 12/2 wire? – Mason Smith Feb 16 at 16:39
  • possibly this system replaces a system that had a mains thermostat, and the mains cable was reused for a 24V thermostat – Jasen Feb 16 at 19:26

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