My home looks like it originally had a whole house heating system that use a main trunk line that runs under the hallway with the vents located at the baseboards. The heating and air system has been updated with flexible pre-insulated duct work that runs to each room individually and the registers are placed in the floor along the outside walls. The blower is in the crawlspace under the house but the unique thing is the new system is using the old trunk line as the cold air return. The trunk is not insulated and should be but before insulated it I was wondering if this configuration is suggested? I love the fact that each room has its own cold air return but both the registers and the cold air return is at the bottom of the floor. It seems like one should be located in the ceiling for proper air circulation? I was thinking about removing the old trunk line and build a chase to place the return in the ceiling. Would this be recommended or is it best just to seal and insulated the main trunk line and leave the return and registers at the floor.