Found system operating with high static pressure with a total of .77 system calls for .5 total. Existing plenum front side of ah is 22" x 13 1/4. May need to cut plenum and extend duct board to have enough room to land a 16" duct. Then run new 16" duct to distribution box, tie in existing 12" and add one all new 10" return duct with stamp face grill.