i live in an 28X81 double wide 7 years ago i had a new outside unit installed and had my old furnace removed. my ac freezes up every year and my heat never quits running during the winter. i had asked that the old duct work be replaced to suite the new unit but it wasnt. i am currently replacing my floor in my kitchen and dining room, when doing this i noticed the main air duct went in to the duct work in my dining room right beneath a register, for years i've noticed that register had a lot of air and the one after it had no air coming from it. for the first three years that i owned the unit i had someone here every summer and winter to look and see why my unit ran constantly. was always told the same thing "insulation isnt good in double wides" well, i don't know anyone with the electric bills i have. fromn what i can figure my duct work is shaped like a rectangle with branches off the main metal duct to the registers in each room. i would like to do away with the branches and have my registers coming from the main metal duct thats shaped like the rectangle which is a duct down each side of the home with cross overs on each end. now my question is where should i put the main air duct, should i add another crossover in the center and put the duct there and maybe add more return ducts to the other rooms. which, by the way there is one return in my living room and thats it, so i know the bedrooms and other spaces need returns as well because over the few past years ive closed off the air vents above the bedroom doors for more privacy for everyone but i'm pretty sure i can add returns in the walls anyway, my question is "where should i place the main air duct?". any suggestions would be very gratefully appreciated.