I am looking to replace my old battery operated thermostat with a new wifi model. There is no "C" common wire connected to my old thermostat. There is a blue wire that is at the thermostat but cut short with no copper exposed. I went up to the attic and took a look at the A/C unit. There was also a wiring diagram that was available. There is no "C" terminal on the unit but there is a "B" terminal which I assume is the right one I should use. I tested the voltage from "R" to "B" and it was about 26.5 Volts (The new thermostat requires 24 volts). There is a wire connected to the the "B" and "Y1" terminal it seems like its the wiring that goes to the outside Unit. Its the other Red wire and white wire. (See photo below.)

I have attached pictures of the wire terminal on the A/C unit in the attic as well as the Wiring diagrams. In the wire terminal picture you will see the extra blue wire wrapped around the main wire and is not used. I would like to use that wire as my "C" common wire. Is it safe to connect the Blue wire to the "B" terminal? Im worried because another cable is already connected to it and its reading 26.5 volts...

Wiring block photo Wire diagram Schematic Diagram

Click for full size

Update 1

First off, big thank you for the help and responses! I really appreciate it!

I went back up and opened the unit. This is what I saw inside. The two wires coming out of the transform have a voltage of about 26.5 volts. I could not find 24 volts anywhere. The transformer says its a 24VAC transformer. Looking at this picture, where do I connect the C wire to? Which wire coming out of the transformer?enter image description here


If you follow the schematic, you'll see that two wires come off the secondary of the transformer; Black/RD and Black/BL. Black/RD goes to CH-4 (fuse), and comes out at CH-5 as Red, then eventually terminates at R. Black/BL goes to splice CH-2, an splits to multiple BL wires. One of these terminates at CH-7, and says to see note 3. Since I can't see note 3, I'm assuming note 3 says something about this wire being used as an auxiliary C.

  • Note 3 states: Accessory Kit Bay24x038 May be used when automatic speed change for heating/cooling operation is desired. (Cut & Strip Blue Leader st CN-7 for conecction of BAY24x038 KIT) – user14188 Jul 26 '13 at 14:09
  • If you're not using the "Accessory Kit", then I'd say it's probably safe to use this wire as a C wire for your thermostat. – Tester101 Jul 27 '13 at 14:17
  • Thanks for the help. I just posted a picture from inside the unit. I tested the voltage from the 2 leads coming out of the transformer and its 26.5 volts. See my update post. Is it safe to connect the "C" wire to one of the cables coming out of the transformer? The yellow or blue wire? – user14188 Jul 27 '13 at 19:58

The B terminal is most likely your heat pump valve changeover (it reverses your AC to become a heat pump). If that's the case, shorting/using that connection will prevent your AC from working.

What you need is the black wire that's coming off the transformer inside the unit, and I can't make out anything in the diagram that shows it connecting to the wiring terminal. You may be able to run another wire from the transformer's neutral to an unused terminal on the panel (it doesn't appear that T or O are being used by either side).

  • The unit is strictly for AC. The heat system is completely separate and uses forced hot water. The "B" terminal seems to be going to the outside AC Unit. – user14188 Jul 26 '13 at 14:13

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