I am hoping to be able to upgrade to a smart thermostat but unfortunately my furnace doesn't have the "easy" option with an obvious C terminal.

My furnace is OLD but it actually has the full 5 wires pulled to it although only 4 of them are in use/connected. All 4 are attached to the appropriately marked terminal at the Thermostat (see picture) but then it's a bit of a rats nest at the furnace so I am trying to figure out where the C wire would go to make the smart thermostat work.

Thermostat Wiring

Is there another wire I can piggy back off of at the furnace to bring the necessary power to the smart thermostat? Does anything in these pictures give you any idea where to attach the blue/C wire at the furnace? Any help you can offer would be much appreciated.

Thanks in advance and feel free to respond if you need a better look at anything from these pictures.

Furnace wires 1 Furnace wires 2 Furnace wires 3 [![Solution?][5]][5]

[5]: https://i.sstatic.net/CD5si.jpgenter image description here

  • For future readers wondering what the solution picture was, connect the blue wire from the thermostat to the bundle with the yellow wire nut. The "white" wire there is beige, but the logic described by @ThreePhaseEel is solid.
    – Carl
    Nov 10, 2021 at 2:58

2 Answers 2


Look for the junction of the thin white wire and the two fat yellow wires -- that's where your C is

Since you have an air conditioner in this system, we can use the wiring going off to the condenser as a clue to find your C wire -- the condenser contactor/control must be connected between Y and C as the thermostat switches R to Y to call for cooling.

Following this logic, we start with the yellow wire in the cable from the thermostat (it's the cable that terminates at the top of the picture), which is connected to the red wire in the cable going to the condenser (the cable that terminates near the bottom of the picture). Since that wire is clearly Y, the other wire in the cable going to the condenser (the white wire, since it's a two-wire cable) must be your C wire, and you can connect the blue wire in the thermostat cable there (after turning the power to the furnace off and stripping the wire end back, of course).

  • Thanks for walking me through that...so if I am understanding you correctly, I can just pull the blue wire into the wire cap right next to it (circled in blue) in the last picture that I just added? Aug 20, 2018 at 23:38
  • @AaronKezele -- yup, that's it! Aug 20, 2018 at 23:43
  • Thanks so much for your help. It comes in tomorrow so I will be sure to update you if/when it's up and running! Aug 21, 2018 at 1:49
  • Just posted a picture of the fully functioning thermostat. Thanks again for the help! Now to figure out how to get it to show the right temp inside (currently +3 degrees) and no obvious way to adjust. Aug 22, 2018 at 5:03

So I am assuming the wire you want to use as a common is the blue wire. Of course you are going to connect the blue to the C terminal at the new t'stat. Now you have to ground your blue wire at the furnace, but in order for you to get it to work you will also have to make sure your one conductor of your 24V transformer is grounded also. The power (red) conductor is attached to the ungrounded side of the transformer and your blue is attached to the grounded side.

You need to verify that the transformer grounded. There is usually a schematic on the door of the furnace that will show you where it is done. If that is not there then you need to trace out the wiring to see if you can find it. If it is not grounded then you will have to ground it yourself. I really can't tell you how since I am not on site.

There is one other option. Most t'stats do not need a common if the stat has a battery back up. The stat will just run off of the batteries and you will need to change it out every six months to a year. You might want to check with the manufacturers help line to make sure it has that capability.

Hope this helps.

  • Thanks! Unfortunately there isn't a schematic on the furnace as far as I can tell and I'm not entirely sure what part is the transformer. I haven't taken out my miltimeter to test wires and whatnot but is that purple/brown thing in the far right of each of the pictures the transformer? It does appear to be grounded to the furnace so would I just need to confirm if there is a terminal that is constantly emitting 24v and then splice the loose blue wire (from the last picture) into that terminal? Aug 20, 2018 at 20:58
  • Didn't realize you can't hit "enter" but the new thermostat doesn't have a battery so I will need to use the blue wire. Thanks again Aug 20, 2018 at 20:59

Not the answer you're looking for? Browse other questions tagged or ask your own question.