I have received 3 bids on furnace and AC replacements. All 3 have sized the units differently and have varying opinions on replacing the duct work. (house built in 1904, with old round metal ducts). How do you know who to believe, is there a formula I could use to calculate proper size of unit? Does old duct work affect how unit functions? What about furnace brands, any preferences? I am looking at Bryant and Carrier the hardest right now.