I was told by a HVAC company that I don't need to enlarge my current duct when replacing it with new carrier infinity 16 seer 5ton unit. The current main duct started at size 30x14 then reduced to approx. 16x8 because it's between floors and blocked by the beam in middle of house holding upstair bedroom. He states that because the main duct is straight from one end of house to the other, when duct gets smaller, air velocity will force it thru to all the vents at other end of the house. What's your thought on this? I don't believe him but like to have some expert opinion. Thank you!