Central air conditioners circulate cool air through a system of supply and return ducts. Supply ducts and registers i.e., openings in the walls, floors, or ceilings covered by grills carry cooled air from the air conditioner to the home..Gas Furnaces. Trane furnaces are innovatively designed with efficiency and precise temperature control in you get cleaner air. Contact a Local Trane Comfort .Whether or not you’ve got ductwork, adding central air is more affordable and less disruptive than you might imagine. Read our guide to installing central AC in an old house at This Old House..Ever wonder how your air conditioner and furnace work together to keep your home comfortable? We walk you through how these two systems work together..