I am getting ready to install a newheat and ac system in the second floor of my home. I have done a manual j and manual d on my home for the second floor. I don't know if i messed up on it or not but it is telling me that i need a 18" round output and a 23" return.( doesn't sound right to me ).I am using a 3.5 ton ac and heating system ( heat pump ) all electric.I just need to know do i put return ducts in all rooms or could i have a 12x12 at one end of house a 12x12 at other end and a 8x8 in the middle.