I went to install a Nest on my upstairs HVAC unit. After installing the Nest I thought it was functioning properly. The next afternoon I came back to the house It was still hot, so looking into this the Heat was running while the AC was running.

Looking at the image, the yellow line running from thermostat is controlling the AC, in this instance it is spliced with the line that runs out to AC condenser outside and then comes back in as the white line connected to Common 24v (C). The Green (G) line is normally connected. The only way I can get the AC to function is by disconnecting the white (W) line from the furnace.

I believe the remedy for this is removing the white line connected to the common 24v (C) coming back from the condenser and move it over to the unused (Y) connector. The Furnace unit is a carrier crusader, and I am not sure how to get the nest to function without having both the AC and heater on.

The unit functioned with the original thermostat. The yellow line runs to the backside, and when the thermostat is switched to cooling, it looks like the RH line is jumpered to RC to power the cooling circuit.

Any Advice would be helpful.

Edit I have found a suggest wiring diagram, and It looks as though I need to have the yellow line return to the mainboard, and then the yellow line from the thermostat need to tie into the same spot on the main board. Thoughts? enter image description here

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  • The wiring in your unit (As described by you) seems to be fine. Are you sure you have the Nest configured properly? Instead of using a twist-on wire connector, you could use the Y terminal on the board to connect the yellow wire from the thermostat to the wire going to the condenser. It should not be required, but it's a common practice. – Tester101 Jun 11 '18 at 10:30
  • The nest auto configures based on what wiring is connected. I believe you are right, but I am not sure why It was wired like this originally however it worked with the original setup. I am going to make that change and see if it works, but I am still open to any other solutions. – TiDoX Jun 11 '18 at 12:18
  • In most older furnaces, the Y terminal on the board is simply a convenient place to connect the thermostat wire, to the condensing unit wire. Can you add a photo showing how you've wired the new thermostat? There's no zone controllers or other systems involved here, right? It's just wiring straight from the thermostat, to the furnace? – Tester101 Jun 11 '18 at 14:47
  • I noticed on the wiring diagram for the controller board the Y line does run to the CPU. You are right though, there is no zone control its just a line from the thermostat to the mainboard. – TiDoX Jun 11 '18 at 16:39
  • Is the Nest calling for both heating and cooling at the same time? (i.e. is there 24V on both Y and W relative to C at the furnace when this malfunction is happening?) – ThreePhaseEel Jun 15 '18 at 8:47

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