Could any of these be the c terminal on my hvac unit? There's already a c wire there, just can't figure out where to connect it.

enter image description here

  • Can you post a photo that shows which cable the red and yellow wires at the bottom go into? – ThreePhaseEel Jan 10 '19 at 1:55
  • I posted a second photo to show where the wires go. – Misty Jan 10 '19 at 2:00

It's the B terminal on your unit

Your unit labels the 24V common as B instead of C. How can I tell? We start with the wire from the thermostat going to the Y terminal on your unit, and then note that since you have central air, the wire from the Y terminal on your indoor unit goes off into the cable to your air conditioner's compressor (outdoor unit). Since the thermostat is applying 24VAC from R to Y (like a light switch) to turn the air conditioning on, then the other end of the compressor contactor's coil must be connected to the 24VAC common for this to work (just like a light fixture needs a switched-hot and a neutral), and that other wire from the compressor connects to the B terminal on your indoor unit.

So take the back-wrapped blue wire from the cable going off to the thermostat (the upper cable in your 2nd photo), strip the insulation off the end, and attach it to the B terminal alongside the wire that's already there. Then you can install your new thermostat, turn everything back on, and enjoy!

| improve this answer | |
  • Then what's W? Second stage heat? And X is first stage? And what the hell is 60,66, and 53? And O? Man this is all fubar. – Mazura Jan 10 '19 at 2:34
  • @Mazura -- not sure -- it appears the W wire from the thermostat goes to the 60 terminal on the indoor unit – ThreePhaseEel Jan 10 '19 at 2:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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