I am having a 4 ton Trane XL16i/TAM7 installed right now. The installers built a long, about 4 feet, return plenum under the AH. It has two 14 inch flex ducts attached to it.The opening in the end of the return plenum is 9"x18" according to them. I saw it and thought it looked too small, expecially since all the estimates I got told me that my two 14 inch ducts were marginal. Is that large enough for a return duct to a 4 ton AH? Thanks for your advice