nestthermostat
My old thermostat wiring looks like this:
So its good I have a C wire and its running on 24v. The hard thing is the fan control. It's a proprietary 3 speed fan – where do I wire those 4 wires to? What is Fan "Com"?
I plan to purchase a Google Nest Thermostat but the compatibility tool is not complete as I have MORE wires than they need for fan control.
The battery is likely depleted and it's pulling a small amount of power to charge the battery. This signal is causing the blower to turn on momentarily. The fix is probably to run a common wire, preferably to the A/C since that's where Nest expects it in a dual transformer configuration (which I assume you have since both Rh and Rc are connected).
Hopefully your A/C control board has a common terminal, and an unused wire. If not, you can use a Venstar Add-A-Wire.
I believe you need to move the red wire at the Nest to the Rh terminal. Rh is 24v from the Heating system. The Rc terminal is 24 volts from the Cooling system. Change that around and see if your issue is fixed.
Best Answer
From the Google Nest Support Page at:
Nest Support Page
It's not clear what the tech will do but I suspect they have some additional hardware that controls the fan.