How large should my furnace intake duct be?
I have a 2500 sq/ft single story home. I want to install a new furnace-AC unit. The system I want to install is a Rheem 125K BTU Furnace. My current intake (return) duct measures 7" X 20". According to my calculations, I'm supposed to be able to move 1560 CFM (for a 125K BTU furnace.) Will the 7" X 20" duct handle this? If not, then what size should I consider?