The fan is set to always on but it's apparent that the fan is off most of the night. When I wake up I never feel air blowing through the vents. When the window is closed during the night CO2 PPM reaches 2000+ in my room while downstairs it's a fraction of that. No air exchange at all.

As far as I can tell the fan always works when it's cooling. And also it randomly sometimes works when it's not cooling.

The furnace is a Carrier model. Thermostat is a Nest unit with fan schedule set to Always.

The Nest thermostat has a fan in motion animation when the fan is clearly off.

Nest wiring: enter image description here

I picked a time when the fan was actually working and set fan schedule to Disabled in the Nest thermostat. The fan immediately turned off. Setting it back to always-on did not immediately turn on the fan and the fan was off even minutes later.

Detaching and reattaching the Nest unit turned on the fan immediately but it was also cooling immediately. So results are unclear.

  • Please edit to include pictures of the wiring at the thermostat and the furnace control.
    – Ecnerwal
    Commented Oct 4, 2023 at 13:05
  • 1
    @JimmyFix-it, CO2 is often measured by those concerned with overall indoor air quality, especially in sleeping quarters.
    – isherwood
    Commented Oct 4, 2023 at 13:16
  • 1
    See also diy.stackexchange.com/questions/182409/…
    – negacao
    Commented Oct 4, 2023 at 13:49
  • I wanna say Nests don't let you do that. When it comes on "randomly" that's the stir setting. Set that to 55m an hour. An AC call runs the fan w/o the green wire doing anything. Jump the green and red, if it runs then your Nest is screwing with you. You could drop the disconnect on the condenser and run the AC, but that's just silly.
    – Mazura
    Commented Oct 5, 2023 at 0:40
  • "Can a Nest thermostat be set to fan only? You can run your system's fan independently of heating or cooling. So your system doesn't need to be actively heating or cooling to run the fan. However, you can only run the fan when your thermostat is set to a temperature mode (like Cool mode, for example)."
    – Mazura
    Commented Oct 5, 2023 at 0:47

2 Answers 2


If the thermostat is calling for "fan always on" and the fan is not always on, it's doubtful that the fan wiring to the thermostat is connected properly, or at all.

It's fairly common for older thermostat wiring to simply have wires to call for heat or cooling, and the furnace or boiler runs fans or pumps from its own local controls, without the thermostat getting to have a say in that.

It's also possible that a wire is broken or not properly connected somewhere; particularly if there's intermittent operation as might happen with a loose connection.

  • 3
    Trimming the G wire on the furnace end fixed the issue. Commented Oct 5, 2023 at 1:52

"Trimming the G wire on the furnace end fixed the issue." John Amrols

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.