Old Thermostat had no C connected to it but furnace has T with wires and there is an extra wire wrapped around at the furnace and at the Thermostat. Can I just attach to T terminal and C on the Nest?furnace terminal [old thermo with wire wrapped unconnected][2]

wire diagram


Based on the diagram, the T terminal should be connected to the wire that controls the outside compressor. That shows that it's synonymous with what we normally call the "C" terminal or wire. You can verify this by making sure there is 24v between the R and T terminals.

Also on the right side of the diagram it shows the low voltage thermostat wires are connected to R and T. T is for sure common. Thanks for the good pictures!

marked up diagram

Also, to keep things more clear for the future, it would probably be a good idea to use the Yellow wire for the Y terminal on the unit and thermostat and use the blue wire for the C/T connection. Those are the "normal" colors and will save you or the next person some time down the road.

And I'm not sure why the Y terminal has two red wires and T has two whites, but be sure and hook those back up correctly. One pair is for the compressor, and the other pair could be a condensate pump.

  • I agree re "T". Your spare is yellow and you have a red wire connected to Y. You don't have two reds going to your thermostat so I'm not sure what you're using for R at the stat. And you are using blue for something, which is often Common. It doesn't matter functionally but it will help you and others to maintain this in future if you use colors as expected. If you use yellow for Common you should label it at both ends. Or change wires around now to go by convention. R, Y, and G should be used in the obvious way at both ends, and the blue wire should be C at the stat and T at the AC.
    – jay613
    Oct 14 '21 at 16:36
  • 1
    @jay613, you can see an old blue wire curled under the two red wires and connecting to the Y terminal. Currently the thermostat is using blue for the Y/compressor terminal which is why I suggesting using the spare yellow for Y and then moving the blue wire to T/C so it matches modern expectations. R, W, and G are all being used correctly.
    – JPhi1618
    Oct 14 '21 at 16:39
  • Thank you so much for the swift and great answers. By the way, there is a condensate pump attached.
    – AlanB
    Oct 14 '21 at 17:25
  • @AlanB no problem. We say thanks with upvotes, and be sure to accept the answer that helped you the most.
    – JPhi1618
    Oct 14 '21 at 17:28

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