I saw the original post about the Carrier 2-stage gas furnace. But somehow I lost it. The question was about the number of wires going to the thermostat. In 1992, we replaced our 18-year-old heat pump with the Carrier Weather Infinity Ultra High Efficiency (whew) gas furnace, #58MVP. I opened the back of the Carrier 7 day programmable thermostat and it has 6 wires.gray, green, yellow, white, red and blue.
This was one of the first variable speed gas units made at that time so my dealer discouraged me from buying one. But I did and I have been very happy with it. I have had about 4 repairs, however. The last one was costly.the DC variable speed motor went out. Otherwise, we have had good performance.
If you want more info about my experience or have something to share..please post or email. Thanks.