So, subject says the gist. I'm curious if this is even a feasible upgrade, and whether or not it would even be worth it.

Has anyone seen a system go this way? I bought the IAQ largely because it was the best you could really get at the time (2008) to control my HP and AH.

Heck, I don't even know if the Nest is capable of controlling everything fully.

Anyone know?

HP and AH are both Trane.

XL15i, 4TWX5030B1 with the 4TEE3F39A1 air handler.

I would imagine an installer would need to remove the back end of the IAQ, and wire the Nest directly into the AH (since I'm pretty sure between the backend unit, and the thermostat, there are only 3 active wires).

Any advice/input?