RGYW stands for red, green yellow, white. These are exactly the colors connected to the ABCD terminals. If it was wired correctly, you may be ready to go right now with no wire changes needed.

Thanks Elmer. Yes, they are color coded, however, they need to be rewired to the RGYW terminals in the furnace, as in the current configuration the Nest throws errors when connected. I’ve found walkthroughs on people rewiring them and it is straightforward, however, my concern is around the fact that the furnace has 2 of each color going into the ABCD terminals. If it was just the 4 wires, it would be straight forward, however since there are 8, I’m wondering if A) there is an opportunity to use the full RGYW terminals (e.g. W1, W2, Rh, Rc, ...) or if I should/can put both reds into Rh, both greens into G1, both whites into W1, ...). Let me know if that makes sense.

I'd say you're at the point of calling an hvac company. Since you recently purchased the house, so as not to waste the service call, let them also spend their time giving you an expert inspection and assessment of your equipment and ducts.

Not all HVAC contractors are comfortable with Nest equipment. Confirm they are before booking the visit.

You might start by posting a photo of the current wiring in the furnace. Model # could be helpful as well. This way someone may be able to better assist you

Thanks Kevin. I thought I included a link to Imgur in the original post, but I’ll add them in-line here. Not very helpful photos, I’ll have to get some better ones tomorrow.

And here is the original thermostat wiring.

I do apologize if I missed it, but did you include the furnace model # somewhere in there? Is this a heat only unit?

The exterior says Bryant Plus 80. Will have to get back up there to get the full model number. Going to do it tomorrow and update y’all.

communicating system, the nest will not work with it.

If you tried to make it work, you may have blown the control board of the furnace.

Congratulations.

It looks like your old thermostat is a Carrier or Bryant communicating thermostat. That is why it has ABCD terminals. Two wires are power and ground, the other two wires are the data lines. You cannot wire the Nest thermostat, or any other conventional thermostat without making wiring changes at the control board. I would not recommend you doing that since you would loose the communication ability. You would also need to run an additional wire to provide the ground connection since WiFi thermostats need a C wire connection.

You need to purchase a Bryant Evolution or Carrier Infinity thermostat. You can get them with or without WiFi. The connection is easy, you then have to go into the service menu and make all the correct set up changes.

I recommend you hire a Bryant or Carrier authorized service dealer to install your thermostat. You could try an independent shop, but they need to understand how your furnace works and install the right thermostat. These thermostats are expensive so expect some sticker shock.

The furnace has legacy thermostat terminals at the top of the board so, in principle, it can be used with the nest. However, you wouldn't use the serial communications block that is currently connected. Use the traditional terminals as outlined in the installation manual (similar to that attached here):

https://www.hvacpartsshop.com/content/bryant-manuals/315AAV-JAV.pdf

Those legacy controls are more or less for someone that doesn't replace the whole system all at once. Sure you can use them, but you defeat the purpose of having a whole functioning communicating system.

A legacy control thermostat will never control this system the way it was designed to run. Oh wait I thought the Nest was smart? Define Smart.

Not to mention there are probably not enough wires to do it if the condenser is a 2 stage system. So then you have the fun job of pulling new wire.

Do what you want, I choose fun. (This thread isn't it.)

Just a question: A, B, C, and D are used but S1 and S2 are not. Is it still 'communicating' if the Signal pair is not used?

To the OP, usually when there are two sets of wires hooked up, its because the 2nd set goes to the compressor outside.

The A, B, C, & D terminals are digital communication terminals that communicate both ways to all equipment connected on this 'network'. If you don't use those terminals (with the proper t-stat) the system can not communicate.

There are preprogrammed algorithmic sequences within the equipment boards & thermostat that optimize the HVAC equipment performance given various parameters as to the temp outside / temp inside / desired cooling temp / desired humidity levels.

Because the system communicates it knows indoor temp / outdoor temp and possibly even more than that, depending on what was included by the manufacturer. (some equipment now is smarter in the sense it doesn't need the communicating attributes in order to perform, because it performs indirectly by controlling refrigerant temp / pressure attributes. [inverter system])

S1/ S2 and in some cases S3 / S4 are sensor wire for optional attached sensors. If you don't have sensors these do nothing. There are also options to set these as alerts (dry contact) in which they alert you to problems with the system, such as condensate overflow pan has water. These offer no communication attributes.

They require a hard wire to whatever sensor they are attached to. Which means you pull more wire. For that reason they are rarely ever used.

I am a Texas Licensed HVAC contractor that serves the Katy, Texas area.

I have the exact setup, dis you figure this out? I need help on this. Did you move both wires, did you get cycling issues even with c-wire?