Details to come
OK, so for anyone who tends to Google things, it’s good to know up front that if you have an ABCD setup Bryant thermostat, you pry either can’t or do not want to use a Nest thermostat instead. First off, it needs to be rewired most likely (unless your installer did something dumb or devious). Bryant uses a serial protocol to communicate between the thermostat and the controller on the furnace. Nest is not compatible with this. However, they will tell you they are. Mostly because wire colors seem to be the primary way people use to identify thermostat wiring. That was great even just 5 years ago. Not anymore. Second, you will get a schpeel about how your Bryan thermostat which costs 3x what the Nest costs is much superior. Much of this is bunk talk by people who get paid to install OEM equipment. However, they aren’t completely wrong. The Nest is still not capable of managing multi-stage controllers the same way as the “official” units are. You will give up some capability to make the Nest work.
Please do your research. Installers generally can make this work. My Bryant unit has the wiring to make the Nest work. However, it disable some of the more energy efficient bits of the furnace. You get some, you lose some. I won’t say you can’t do it. I’ll just say you pry shouldn’t for some of the newer multi-stage serial communication furnace controllers.
I know this cause I just failed to install one. My OEM installer came out, we talked about it, and I decided to stick with the clunky older thermostat so I could keep some of the more energy efficient bits of functionality (and the humidifier, which couldn’t be hooked up, wrong # of wires).