I'm thinking about remodeling a basement space occupied in large part by a gas-powered forced air furnace and its associated ducts and tentacles. Most of them are high enough towards the ceiling not to be too bothersome, except for one of the three return air ducts.
The other two return air ducts run in parallel with the joists, so they are embedded within and are essentially "in" the ceiling. The third return air duct, our problem child, runs perpendicular to the joists, so understandably it needs to be lower.
HOWEVER, for reasons unclear to me, it also has an incline: while the end connecting to the intake grille is flush with the bottom of the ceiling joists, the end near the furnace is almost a foot lower. The duct itself is consistently 32" x 8" and runs 11 feet.
My question is: Is there something inherent to the design of the return duct that absolutely requires it to have an incline? Or can I lift it and straighten it out, so that the headroom directly under it is a constant height? My confusion is further fueled by the fact that the other two return air ducts do not have an incline -- just this one.