Hopefully this will finally solve the issue with my Nest. I bought a Nest 3rd Gen Learning Thermostat and followed the instructions to the tee. I have a two transformer system (Forced Air A/C and Forced Air Gas Furnace) configured as follows:

  • From my furnace

    Power wire (black, connected to Rh) Heating wire (white, connected to W1)

  • From my A/C AHU

    Power wire (red, connected to Rc) Fan wire (green, connected to G) Cooling wire (yellow, connected to Y1) 24V common wire (blue, connected to C)

I started up the thermostat and tested the furnace and this would happen every single time:

  • Furnace starts up
  • Burner flame ignites
  • Burner flame kicks off right as the fan is supposed to turn on
  • Fan turns on (blowing cool air)

I began (with Nest's support) unplugging wires from the A/C until I only had the furnace wires connected. Only then did the furnace run properly. Nest tried telling me it's because I have a two transformer system and I would need isolation relays so everything can run properly. I thought that the Nest 3rd Gen works with two transformer systems, so this seems wrong.

Has anyone else run into this problem? Will relays for the fan, cooling, and heating solve this problem? Is there a less costly solution?



Nest Wiring

Old Thermostat Wiring

  • Can you post photos of the wiring at the furnace/AHU end of the setup? – ThreePhaseEel Jan 8 at 2:35
  • Also, is the Nest dropping its call for heat (24VAC from W to C at the furnace) when the furnace burners go out? And do you somehow have your DX coil (evaporator coil) in a separate air handler instead of having it fitted to your furnace? – ThreePhaseEel Jan 8 at 2:42
  • Hi ThreePhaseEel, I'll have to get the pictures tomorrow. There is no C connected to the furnace, it comes from the AHU. My evaporator coil is in a separate AHU instead of fitted into the furnace. The call for heat isn't dropped, it was still trying after the burner went out. – M. Gentile Jan 8 at 4:46
  • Is the Nest calling for fan from the AHU while calling for heat? (24VAC from G to C at the AHU) – ThreePhaseEel Jan 8 at 12:35
  • I set it up using “Nest Pro” instead of “Homeowner” to make sure that the fan wasn’t activated by a call for heating. – M. Gentile Jan 8 at 12:38

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