While you may be looking to keep warm in the colder months, you may also want to enjoy air conditioning during the warmer months in which case a combination AC plus furnace system can be used. If a natural gas connection is available, when forced air central air systems are installed, they are paired with a natural gas furnace if the entire HVAC system is being replaced. If you already have an electric heating system in place, a central air conditioning system should be able to be installed alongside it, though more ductwork may be necessary.