We are thinking about getting a central air system for our home. The home is 3500 sq. ft. two story. Although 3 bedrooms and 3 other rooms are shut off, (doors and vents closed). Our Lennox furnace was purchased just last winter. How big of a unit would we need to just be comfortable, (don't need to be freezing). Plenty of large shade trees around the house. We live in southern Ohio so summers can be very hot and humid.
Thanks