When I had a new furnace installed 10 years ago, I needed to install return vents into the upstairs bedrooms of my Cape Cod style home. The HVAC company ran a single 12" round duct from the main trunk across the basement ceiling, up through a bedroom closet, and up to the second floor. From there, the one 12" duct branches off into two bedrooms.

I'm trying to figure out if there is any way to conceal the basement duct into the ceiling without throwing off the flow of air from the upstairs bedrooms.

The best option I've been given so far would be to pan off the joist bay to get the most volume of airflow as possible. If my math is correct, a sealed and covered joist bay still would be less than the volume of air in a 12" round duct. However, I don't think I'm getting full flow out of the current set up because the portion of 12" flex that goes from the rigid duct to the main trunk was compressed into the bay to squeeze above the main support beam to the house.

If I was to pan off the bay, I can't figure out how to connect that space cleanly to the 12" flex that's already there or to the return main.

Thanks,
Logan

12" duct has 113 sq. in. Your joist spacing X joist depth=? Pretty common to use dead joist space as a return but you could use as a supply. Put the duct into the vertical space, use orange fire block foam to close off the side holes(should have been done anyway). Fashion a plate from flashing to cover the chase but not the duct. Use flashing across the duct dead space, UL approved metal tape to seal the flashing and then duct mastic.

The bay dimensions are 14x7.25, so that gives me 101 square inches. If I understand what you’re saying, it sounds like you are explaining how to attach the vertical duct into the bay I want to close off. I know how to make that connection. I’m not sure how to connect the bay I want to cover to the main return trunk that would be to the left in the third picture above. Right now, there is flex duct, but I can see how I would connect the bay I want to seal off.

The other horizontal return runs between the joists in the ceiling were panned off like you describe. But I can’t see how they connect to the main trunk. Would I have to drop the return trunk to make that connection?