So the 2 furnaces are next to each other in the basement/lower level? IF so, on simple fix might be to leak-lag the furnaces and tie the ductwork together and add back draft dampers. I suspect one furnace and the 5 ton AC can carry the whole building. I think one pro on here once said they've never seen a undersized furnace. You'd have to be massively undersized not a ocarry a space over a 24 hour period.
One thing to remember, the larger the home, comparatively more thermal mass and less surface area to volume ratio. Also being 3 stories, you have a proportionally smaller roof as well compare ot a signle or 2 story home. A 4000 sqft home will never use nearly twice the heating and cooling as a similarly designed 2000 sqft home, and far less than 4X a 1000sqft home. A typcial 1600sqft home might onl need 2 tons and 50k BTU's, while 4 tons and 100k BTU's can carry many 4000 sqft homes.
To Motoguy...Houston residential construction is typically slab-on-grade, so there is no basement. OP's systems are either in a closet or in his attic.
Originally Posted by coolinhouston
Originally Posted by Shophound
DO NOT ABANDON SETBACK STRATEGY or you are likely to have some real latent problems. You probably really need the long pulls.