I have an old (1916) 2 story house with gas furnace located in a basement utility room. I have no cold air returns on the second floor. I have 3 cold air returns in the main floor (living room, entrance hall and dining room). Only the return in the living room is connected to the furnace air intake through ductwork. The return in the dining room is sealed and the return in the entrance hall is open to the space between the floor joists and the sheet rock ceiling in the basement. The return duct from the living room connects to the furnace air intake and the furnace air intake is also connected to a grill in the utility room wall and pulls cold air from the basement. The basement is finished but it has a hole in the wall sheetrock (exterior wall) to provide access to a sewer cleanout. As a result, the furnace draws primarily cold air from the basement and wall cavity and only a portion of conditioned living space air. This reduces the efficency of my furnace.

I am in process of sealing my basement and using it as an entertainment room. Do I need to redo the ductwork to connect up the 2 air returns?. This may be difficult due to the direction of the floor joists and low ceilings. One possibility I am thinking about is to seal the basement space and connect the 2 returns to the basement by installing flexible ducts from the floor returns to grills in the basement ceiling so that the basement serves as part of the conditioned space thus eliminating the need for return air ducts in the basement.

Does anyone have any thoughts on this approach? Will it result is noise or some other performance problem.