Wiring a Nest Thermostat to a Carrier Furnace

UPDATE 7/10/2016: I've abandoned the Nest in favor of the Carrier Infinity.

You removed your Carrier Infinity thermostat and find four wires labeled A,B,C,D. These won't work with the nest. To make it work, the furnace will need to be opened to access the furnace motherboard. Then, those four wires must be removed from a plug harness, and reconnected to the terminals on the furnace motherboard. Fortunately, those terminals will be labeled, and will correspond to the Nest wiring harness.

The Carrier 58MVC will work with the four wires reconnected to Y1, G, W1 and Rh. However, if you want to take full advantage of multi-stage heating and cooling, then you'll need to run additional thermostat wire. Contact an electrician or HVAC professional for assistance. Many of your questions are going to be answered in two places:

1. This forum thread.
2. And this forum thread.
3. The Nest pro installer guide.

Nest does not support Carrier furnaces wired with the A,B,C,D layout and requires that you contact a pro installer to engage in any rewiring. A misplaced connection could cause substantial damage to components in your HVAC system.