I have 2 sets of HVAC, one of which I started installing Google Learning Thermostat on, and searched far and wide for Nest related information on the Bryant / Carrier Evolution series. Could find nothing that says they were compatible, and some Google Nest experts claiming that they were NOT compatible.
After looking at the Bryant diagrams, and some photos, I found that this system, normally a 4-wire parallel interconnect (all units on a bus) which has 2 power wires, and a transmit and receive. I thought, that looks familiar, and then found that someone else had identified this as RS485 current loop bus, and that it might even support MODBUS (still not sure) and even found some source code out there.
Opening up the cabinet, I found the controller on the top had both 4-wire and legacy connection strips, with the same annotations as were on the Nest.
I had already hooked up the Nest at the thermostat end, and it woke up, wasn't feeling very energetic, and I found that it was possible that I needed a Common (C) wire, for which I borrowed a Blue wire from the remaining 4 wires at the Nest, and it was now happily chatting on the Wifi, downloading updates, tra la la.
I shut off the HVAC with the mandatory power switch on the wall next to it (why all states don't mandate this? Foolishness), but the cabinet cover had already done that for me, see black intrusion switch on the top. Seems that leaving this cabinet open could cause CO2 to leak from the HVAC, but I wasn't impressed by the door seal, far from gas tight, a bit more like wind tight.
My system was a bit more fangled, since it included an inline water sensor in the HVAC bathtub tray (what else do you call it?) which would cut off thermostat power if it got wet - it's in series to the thermostat ground (black), so you can follow the orange, red, black, green (off photo).
The RS485 is the 4-terminal green block on the right hand side. Removed one set of wires by elimination, and moved them to the old-style terminals at the top. It worked.
Now the question is, how to connect RS485 MODBUS to Nest?
The downstairs HVAC has 2 thermostats, 2 dampers, and AC/Gas heating (heat pump one day).
It seems there might be a way using Zigbee to RS485, working on researching that. Anyone else tried this yet? I mean, I could buy a board and hack the programming, or maybe there's enough smarts in the Nest device to speak MODBUS (a bit like the old AT MODEM commands, with some reliability stuff added). The point being, that there is so much information available in my HVAC, as others have posted, to measure efficiency and changes to insulation, that would warrent some sort of officially supported system. I would love to have variable fan speed control, and ownership of vacation temperatures by the basement thermostat. Also would love pre-emptive cooling, where, if the temperature outside is rising rapidly in the summer morning, that the AC starts removing the heat before it builds up in the mid-afternoon and requires grid battery backup to kick in. Or even to re-hookup the fresh air ventilation system which was disabled, that would kill the AC and open the outdoor air upstairs to cool the house around sunset if the temperature outside was lower than upstairs, which it is about half the time. Currently I have to manually open 2-4 sets of windows and/or skylights.
On the other hand, not much on photos is available on the web, possibly due to takedown requests? A handful of sites no longer own their domains. Lets see if this post lasts.
https://www.doityourself.com/forum/thermostatic-controls/636995-bryant-evolution-nest-thermostat.htm... shows a similar quest, and talks about using EcoBee4 as an option, but not sure how that hooks up to a smart HVAC.
