Yep. Blue is 24V common, or C in your air conditioner. So the large blue wire with the red thermostat wire running to it is C. Just add your C wire to that, and the R wire that comes from that unit goes to Rc. You’d probably be best off running the C wire all the way to the thermostat and foregoing the C adapter, if that’s not too hard, Otherwise just hook it up with the adapter like the diagram I made earlier.
Thank you Speadie. I have a couple of questions. First. what do I do with the red wire that’s connected to the blue wire in the AC? Second, I still have only one red coming from the thremostat. Does this go to the furnace with no red wire going to the AC? Do I need two red wires to the thermostat along with the addition of a blue wire (i.e. one yellow, one green, one blue, one white and two reds)? Thanks again!
C is common, that red wire is doing something, so just leave it there when you add your C wire to it. C can have several devices attached to it without an issue.
If you only have Y, G, and R wire going to the thermostat, you would need at least 2 additional wires (Rh, and W). You might be best suited by running some new 6 wire thermostat wire to the place you wish to install this thermostat, with Rc, C, G, Y, Rh, and W wires. Send the Rc, C, G, and Y wires to the A/C and the Rh and W wires to the furnace zone control.