Why isnt my furnace fan running when the thermostat is calling for it?
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:
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.
Question from user John Amrols at stackexchange
Answer:
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.
Answer from user Ecnerwal at stackexchange
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:
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.
Question from user John Amrols at stackexchange
Answer:
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.
Answer from user Ecnerwal at stackexchange