I'm getting ready to have a 2 stage AC/furnace installed in my downstairs. To prepare, I bought a couple of ecobee thermostats (one for the new 2 stage system, one for the old 1-stage system upstairs). My AC guy is telling me the ecobee needs more wires than I have ran to my panel.

I've been looking at documentation online and it seems my existing wiring (5 wire) will support a 1-stage system fine but not 2-stage. Am I reading this correctly? It seems the thermostat they are going to install somehow communicates to the 2stage system without the need for the additional wiring at the panel, and I'm not sure why the ecobee can't do that too.

Pic of my existing thermostat and wiring below.

Some of them like nest for example uses what people in the industry call "power stealing" to charge the battery, maintain the memory and run the display and generally can get by with one less wire in most standard configurations. The ecobee (and others) use a true 24V power circuit for these things and require an additional wire. As I remember the ecobee set up for 2 stage usually takes 6, 7 or 8 wires depending on the furnace control board and various combos of 2 stage heating/cooling. As I remember a 2 stage heat pump and 2 stage furnance combo needs the most wires. Most ecobees come with some kind of power extender to supply an extra wire if the current system doesn't have one. That frees up one of the other wires currently in the system. Have used the extender on some single stage systems just needing one more wire successfully but never had to look into it for a 2 stage system. Don't know if that is an option or not or gets you what you need for your configuration.

I can only add a little here.

Lennox systems have i + and i- wires that communicate info. Ecobee can't decipher that and you'd need rewire control boards (they have the pins) to speak more directly to ecobee with stage 1 and stage 2.

My Trane 2 stage heat pump uses 9 wires.

Ecobee and Nest 3rd gen can control multi stage heat and cooling. But you lose the diagnostic intelligence or other potential features supported by their proprietary data links.

When we moved into our home there was some piece of crap Lennox thermostat that was probably $700 new and it could barely connect to WiFi, was inconsistent and all around a piece of garbage.

I swapped to Nest-E and had no issues. Note: Nest-E is more limited and can only handle either 2 stage heat or cooling but not both.