i have this air handler TUD1C080A9H4 and i was suppose to install the return grill etc. due to wall repairs etc. my question is this will a 16x25 return be too small for a stand up furnace ? thats seems like the only one that would fit for the stand up unit. we didnt think about this when we did the box for the furnace to sit on. if i wanted to put a 20 x 25 return grille i would need to pick the unit up about 4 inches taller, which will affect the plenum because its already hitting the roof as it sits already. not much room up there to pick the furnace up a couple of inches.