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.
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.