what size unit do we need to cool 1800 sq. ft.? We currently have a 2 1/2 ton gas pack but it is only vented to cool and heat the downstairs. we are wondering can we expand the ductwork upstairs, so that we don't have to freeze/sweat when we are upstairs. we are working on a very tight budget and would like to do this as inexpensively as possible and still have it work reasonably well.