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…
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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?
Best Answer
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 auxiliaryC.