I installed a Nest thermostat. The heating and cooling work fine. I want to be able to run the fan only. I live in a two-story condo, and the temperature differences between the floors are dramatic. Plus, I want to filter out allergens during spring.
I've scoured these pages for solutions. The fan-only option appears in the UI but doesn't run. I tried the fan-only setting in Heat, Cool, and Off modes. I tested all the equipment options, including heating, cooling, and fan. I updated the software and restarted it. The fan works fine when heating or cooling, just not by itself.
Now, I think the wiring is wrong. I know the fan worked with the old thermostat. Attached is a picture of the old wiring, the new wiring, and some settings. I'm willing to offer someone $100 to diagnose the problem! My objective is to run the fan independently of the heating and cooling. 
I'm no expert, but I find it easier to start the fan from the home app on my phone. But I just figured out how to use it on the device itself. I labelled the pictures with instructions. Hopefully this helps.
Thank you! The color is Sand. I ordered it in November and installed it in February 2024. I ran a new 8 wire from my furnace to the thermostat. Right now, it is only controlling the furnace (and fan). I use a swamp cooler in the summer. But I would eventually like to add a UV light and humidifier to my furnace and want to be ready for that (and if I ever switch to an air conditioner). In the old wiring, one of the wires was damaged somewhere in the wall (which was another reason I went ahead and ran a new set of wires). Because I DON'T have air conditioning, I don't have 2 of the wires connected (see pic). I know for me, connecting the Y (yellow) wire was causing issues, and I think it's because I don't have air conditioning.
I figured out how to use the Nest touchscreen when I was trying to see if I could help you. 😬 It's just so easy on the phone. I'd like to hear how you figure it out.
