Update to Nest Versus Carrier 58MVC Variable Speed Furnace

I previously wrote about installing a Nest thermostat with my Carrier furnace, which has variable speed. After over a year of using Nest, I've gone back to the Infinity thermostat that came with the Carrier furnace. The Nest thermostat does not support variable speeds. By wiring the Nest thermostat in the manner that I did, it caused the furnace to omit that variable speed feature, and instead run like an ordinary 2-stage furnace. I thought this was going to be adequate, but over time, I came to appreciate that it really was not very efficient, and thus defeated the whole purpose of the Nest. By running in two stages, the house was either too hot or too cold at times, when compared to the previous ability to run in variable speeds. Moreover, the Infinity thermostat has the ability to reach proprietary sensors that could alert me to system malfunctions. My system did malfunction when ice froze over the furnace external exhaust, but the Nest couldn't tell me about that, whereas the Infinity could. After bringing out the repair professional and talking more, we decided that the Nest was more of a problem than a benefit, as I could have avoided the service call if I had just had the Infinity to begin with. Moreover, the variable speed mode could have prevented the ice freeze, since it probably would have continued operating at a lower power, preventing the freeze, whereas the Nest simply shuts the furnace off, allowing an opportunity to freeze.

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.