I have just had a Unico/Carrier high speed A/C system installed in my older, brick Cape Cod house. The system uses insulated 4-inch round ductwork. It's just for cooling but I understand this kind of system can also be used for heating. That said, I'm wondering why TOH used those huge metal ducts in the Boston house renovation that's now underway. It would seem that a system that uses much smaller ductwork would be much better for a retrofit in terms of ease of installation. In my job I lost about one half of one square feet of space in only one closet, with the rest of the ducts run through joist spaces, walls, etc.