[Gatorac]

It sounds like you have what's called a "hybrid" system. A heat pump with a gas furnace. A heat pump will cost less to run then a gas furnace down to about 40 degrees outside temperature. The system has an outdoor thermostat that will run the heat pump until it gets too cold for it to be effective and then lets the furnace take over. The previous owners were smart and made a wise investment that you get to take advantage of.

I'm only an air conditioning contractor with about 22 years in the business so I may be wrong. Give me the model numbers of the equipment and I might be a little more confident.;-)