I am refinishing the basement and need two additional air ducts in the bedroom and bathroom from the existing central air conditioning plenum. I need suggestions and/or diagrams on the proper assembly. I want to use rigid ducting and am looking at possibly 4" or 6" diameter with the total length of both locations of 8 linear feet.
Use a 5" or 6" diameter duct. You need to buy an adapter from the 5" duct (for instance) that will fit a standard 4x10 or 3x12 floor register. The adapter or boot is secured to framing in the wall or a square hole cut in the floor. The other end which goes into the plenum is fitted through a proper sized hole in the plenum, notched about 1" x 1" to form tabs. Each alternate tab is bent into the inside of the plenum and its alternate is bent onto the outside. Space out 3 screws around the duct on the outside and screw these tabs into the plenum. Wrap proper foil adhesive tape (not duct tape) around the outside of the 5" duct and the plenum to seal off any air leaks. Also tape the joints with this tape as well.
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