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.
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Can you post a photo that shows which cable the red and yellow wires at the bottom go into? – ThreePhaseEel Jan 10 '19 at 01:55
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I posted a second photo to show where the wires go. – Misty Jan 10 '19 at 02:00
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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!
ThreePhaseEel
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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 02:34
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@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 02:36