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I just moved into a new apartment with HVAC, and I'm looking to control the thermostat with a few Phidgets (I had already invested in that ecosystem, so I just needed a couple of relays).

Anyway, when I connect the R and G wires together with my relay, the system turns on both the fan and the compressor (the letter behind the green wire is indeed labelled G). I'm guessing this isn't normal; how would I go about troubleshooting this? I haven't had the chance to turn the Fan switch to "On" on the original thermostat to verify its operation.

Shorting the blue wire (labelled Y in the photo) and R turns on the compressor + fan as well, which is expected.

thermostat wiring

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  • Have fun with that. I had to do this on an HVAC system at work and the wire colors and their function seem to vary from installer to installer and vendor to vendor. It took me a few hours of research to figure it out. Bottom line is you need the manual to your HVAC system to see what wires control the heat/cool function and which wires control the burner, fan, and compressor.
    – Mister Tea
    Commented Sep 26, 2016 at 20:12

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You're going to have to go look in the air handler, to determine how the control wiring is connected. The thermostat is nothing but a fancy switch, so it doesn't tell you much about how the system is wired.

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