I’m in Colorado, elevation 8700 ft. Very little air conditioning here, but with the rising temperatures I’m considering adding central air to my central heating. My lower level in no way needs a/c especially with our very low humidity. I’ve had two estimates that figured 4 ton units which I understand might be needed for my total area, but no way I need that much to cool 1700 sq ft. Currently I have two 10,000 btu portable units and rarely need the second. I would prefer central over these portable units because of the noise and venting. I understand it becomes a furnace fan volume issue when I shut off the lower level but otherwise it’ll be an ice box if I don’t. With our kids grown, the lower level vents are closed in winter also. The Manual J calculator doesn’t allow me to pick a location even close to my area or elevation. Is there a way to make this work?