Step 21: Ductwork - Install a New Cold Air Return
The main cold air return duct passes straight through the workshop area of my renovation. I decided to install a vent in the end of the duct, as far from sources of sawdust as possible. Before enclosing the duct in a soffit (described later), I did the following:
First, measure the area where you want the vent to be. There are many different sizes of vent covers available, just pick one that first best. With the vent in hand, decide how large the opening in the duct should be. In my case, it was the area in the center, not the outside edges! Transfer those dimensions to the duct using a permanent marker.
You will need to install a small duct that runs just from the edge of the main duct, to the vent (which, in turn, rests on top of the drywall). So that you have something to attach that duct onto, you'll be cutting tabs into the main duct. Inside the rectangle you drew on the duct, draw a smaller one about 1" smaller on each side. Then, draw lines between each of the four corners.
With the drill, cut a hole on each of the corners of the inner rectangle. Then, with the shears or nibbly cutter (whichever works best for you), cut out the smaller inner rectangle. Then, cut along the diagonal lines that link the corners of the two rectangles. Fold the tabs outwards as neatly as possible, and try for a clean 90 degree bend.
Cut a small piece of sheet metal to act as a bridge between the hole in the main duct, and the vent. It should be made to fit tightly around the outside of the tabs, and wrap all the way around. You may want to leave this step until you're ready to install the drywall.
Now, go off and install the soffit around the ductwork as described later in this instructable. When it comes time to drywall the section where the vent will be, transfer the dimensions of the vent opening to the drywall surface. Then, cut out the opening. Fit the drywall in place, and slide the small piece of ducting you made earlier into place. Everything should line up properly. Screw on the drywall.
Using aluminum tape, attach the tabs to the inside of the small piece of ducting. Try to keep the tape as smooth as possible, and be sure to seal up any air gaps. Finally, attach the vent cover on top of the vent hole.