I see some folks are not aware of what a modulating furnace is so let me explain.
A traditional (1 stage) furnace had heat on or off.
2 Stage furnace has Heat Low (usually 65%) and Heat High (100%)
Modulating furnace has many stages, and the real advantage here is that it can go as low as 30%. This is not just the fan that spins slower but the burners as well. It has a variable gas valve and so on.
To achieve good comfort, you want long heat cycles. To achieve that, the modulated furnace needs a “special” thermostat. The main thing it is looking for is the required temperature rise (current temp vs target). It can also check outside temps, humidity levels etc. The other main advantage of the modulated furnaces is that they can solve the problem of undersized (old) ducts. Units have pressure sensors and can regulate speed/fire if static pressure is too high and so on (regardless of what the thermostat wants).
Same thing with cold and A/C.
The sad part is, these HVAC manufacturers are years if not decades behind and each pushes each own protocol, although there is something called ClimaTalk but that is just not going anywhere. Their “smart thermostat” are smart, but lone woofs - impossible to integrate them with whole house smart systems. There is no open APIs and so on. And there is a reason for that. They want to sell you zoning systems, their sensors and so on.
I am actually negotiating new furnace install these days. Looking at the York Affinity line. Support from York is poor, most don’t understand when we ask them for API. Sniffed some packets and it looks like it is using JSON via Amazon servers. Unfortunately I am not that good of a twicker to be able to reverse engineer it. But even if I was, as all things cloud, what happens in 5 years with that service?