I am in the early stages of hiring someone to install an a/c - heating unit in my two story house. I currently only have forced heating ducts in my house. My 1965 house was not built with an air conditioner. Only a heating unit. Overtime I have changed about 30% of the ducts using a thicker gauge duct and sealing all ducts with silver backed adhesive tape and topping that with cloth tape with a pasty glue on all joints meaning it will never leak or come apart. I used to be an insulator so I have some experience in doing this. I am thinking the size ducts I will actually need will probably be dependant on the size unit I buy for my house. My neighbor just had a 4 ton unit installed in his attic and he has 1400 sq ft. I have 1950 sq ft and I am thinking 5 ton just because of increased sq footage. I still have a lot to learn. My ducting goes from a 24" plenum attached above the 1st floor heating unit to 8" horizontal ducts reducing to 6" ducts for 1st floor rooms. There are two 8" ducts coming out of plenum and one is used for 1st floor rooms farther away from unit and the other 8" duct is sending air to the vertical reducer headed upstairs which reduces to a 6" duct for the two rooms upstairs. The current heater I have says its input is 100,000 btu/hr and the bonnet capacity is 80,000 btu/hr. I am guessing this would have helped determine size of the duct back then. Is it possible to do the size layout I need by myself,maybe getting it verified by someone in this field and then get a good installer to finish the job? I am retired, remodeled most of my house over the last 25 years so I think this is something I could do. I have been in my attic many times insulating ,running new ducts,etc.... I would appreciate any input. Thanks