I am trying to hook up a new wi-fi thermostat. I currently have 4 wires running, but there are many unsued that are terminated in the jacket of the wire. No issues there. However, when I get up to my furnace, I don't see a connection for a C wire. The furnace is old, however, I am faily confident there should be a way. Any help would be awesome






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2 Answers 2


Take the blue wire from the cable to the thermostat, nut it with the two blue wires in the foreground, done

The two blue wires in the foreground of your first two wiring shots are both C wires -- the thick is the C wire coming from the furnace, while the thin one is the C wire returning from the air conditioner compressor (it's the one that goes into the cable where the yellow wire was the only other wire used). So, you can simply strip the end of the blue wire in the cable to the thermostat and include it in the junction with the two existing blue wires up front.

  • @KevinRMcGuire -- we thank people around here by upvoting and accepting their answers :) Apr 2, 2018 at 2:45

Every 24V transformer has 2 wires on the low voltage side: R and C.

Even if neither of them was marked at all, you can follow the two wires and find an opportune point to tap/splice to one of them.

If you don't know which one is C, try one. If it turns out to be R, that will do nothing to your thermostat. (it won't power up, but it won't fry either). The thermostat's purpose for wanting C is that it wants to power itself with the 24V supply between R and C. If you accidentally connect it to R and R instead, that will be 0 volts - which is harmless. In that case, it will not power up, and you try the other wire.

  • So ur saying to tap into the incoming power or the outgoing from furnace? Glad to know I can do it as I'm buying an older place with original oil furnace and concerns it won't have a terminal. Just got the Honeywell for half off on prime day, pretty good deal! Oct 14, 2020 at 3:56

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