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I recently bought a Nest Thermostat and I've had nothing but trouble with it. The main symptom is the fan and the AC compressor won't run at the same time. Power only comes through the green wire, not the yellow. So I have been running AC with no fan just to make it through the hot summer days.

I think I have narrowed down my problems to not having a common wire connected. Luckily, I have an 18/5 bundle running to the thermostat so I was able to connect the common wire. However, the nest reported no power to common (E79). I started to look into the HVAC unit manual and I ran across the following sentence.

Do not use any type of power-stealing thermostat. Unit control problems may result.

I know Nest thermostats can be power-stealing. Is this a show stopper for me? Do I need to revert back to a "dumb" thermostat?

For Reference...

  • Can you measure 24VAC between R and C at the thermostat? – ThreePhaseEel Jul 28 at 17:44
  • I'm getting .3 between R and C and 27.5 between R and everything else... – Devin Trowbridge Jul 28 at 17:55
  • Can you post photos of how the control junction box in your rooftop unit is wired, then? – ThreePhaseEel Jul 28 at 18:03
  • Here is a link to a photo album imgur.com/a/s0tstVT with some images of the actual unit as well as a screen grab of figure 14 of the install manual. – Devin Trowbridge Jul 28 at 18:37
  • Is there a wiring junction in the middle of your thermostat run? It seems like the cable at the package unit end is something like an 18/7 or 18/8, not an 18/5... – ThreePhaseEel Jul 28 at 19:09
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Yes the PY4G unit is compatible with the Nest 3rd Gen Thermostat. I'm guessing that the quote from the manual about power-stealing is to ensure users use the common wire and don't rely on smart thermostats' power-stealing.

My issue was that I had 18/8 coming from the outdoor unit that met with 18/5, but only 4 wires {Y-G-W-R} were connected. The junction was hidden behind the aluminum flashing that protects the main duct before it enters the crawlspace. This is why I didn't see it when I was in my crawl.

I connected the blue wires, got a good reading on the multimeter, the Nest Thermostat no longer gives an error, and my HVAC is working again. Shoutout to @ThreePhaseEel for the help!

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