Carrier Infinity Thermostat - install question (HVAC heat pump integrate with existing GAS heat)?

You would be better off to buy the good stuff from a Carrier Dealer rather than Sears. The salesman should have told you that the Infinity control requires a new Infintiy furnace and will likely not work well (and certainly not get the desired efficiency) with your older furnace. When properly matched, the thermostat serves as the microprocessor unit that polls the connected loads and configures itself just as a plug and play component does on newer computer systems. I would be really hesitant to buy a system like this from Sears but you will love it if you get it installed by someone who knows what he/she is doing.