Why is the indoor A/C Blower randomly turning on and off only with Nest


I recently purchased a Nest Thermostat (3rd Generation). My old thermostat was working perfectly. I wired Nest the Same exact wiring as the old thermostat which is : Y1 W1 G RH RC. With Nest, the indoor fan blower randomly turns off for 3-10 seconds, then back on for few to several minutes. I replaced my indoor fan, thinking its a blower issue and still doing the same thing. I replaced my old thermostat, its working perfectly, but only when i use Nest, the Blower randomly shuts off then on..

Best Answer

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.