Auto fan stopped working

[Zarneywoop]

Hello, I have an older (Nest 2?) system that has worked fine over the past several years, however an issue just came to light where the auto fan does not work. I can turn just the fan on manually (with or without the heating being on), but the auto function now doesn't work.

Any ideas & could it be a need for a new nest?

[CoolingWizard]

@Zarneywoop,

if you have a system that’s been working and when you turn onto heat mode and say you wanna raise it 3° the blower fan does not come on. This is likely a problem in your furnace and not the thermostat. Since you can manually turn on the blower from the thermostat we know the blower control relay is working.

Now with a furnace, the blower does not turn on automatically at first. Typically what happens is the heater turns on and the system waits until it receives a signal from the heating chamber that has reached 200° and then it turns the blower fan on. If that sensor is not working, it will not allow the blower fan to come on and intentionally what will happen is your heat box will overheat and shut the gas off. You need to have your heater system checked. 

AC Cooling Wizard

[Zarneywoop]

Hello CoolingWizard, thanks for your message. This system uses hot water pumped through a heat exchanger (no furnace). It's not great, but works. The fan has three speeds (Hi/Med/Low) and when working, the 'heat on' condition immediately starts the water pump and fan (low). When either the fan is on by itself or the A/C it's the 'Hi" speed setting which still works in either of those modes. It's as if just the low fan speed setting doesn't work? I'd love to post the wiring/board diagram, but don't see how to send images.