Originally posted by rabadger
You need to ask yourself when the windows, doors and insulation will be done. If you update the equipment with new that is sized for the current conditions and then do windows, doors walls and attic the new equipment could end up oversized three or four years down the road. You need two sets of load calculations done. One set for what you have and another set for what it will be after windows, doors, and insulation. Without that info you are just guessing. You must see if the old equipment can be used with a zoning system and ducts to the addition. If it can be done then do the addition and windows, doors and insulation. When you are finished put in the right equipment to match the load. When you have them do the load calculations have them done room by room. That will tell you other potential problems that you may have with the duct work.