I am remodeling our bathroom and I want to remove all firdowns. There is a return air plenum built into the firdown. I was thinking of using 3.25x10 inch rectangular duct to go inside the wall however this will require removing the upper plate in the wall to allow the duct to enter the attic. I am concerned about the effect of this on wall integrity.Does anyone have suggestions or best practices regarding this?
The return duct to the furnace is 14". It was supporting (3) 8 inch ducts. One for the living room, one for a bedroom and one for an office. I am replacing them with (3) 3.25x10 rectangular duct with 7 inch caps that will connect to a duct box to tie all to the 14 inch duct.