How to Modify a Cold Air Return Duct
- 1). Locate all existing return duct openings. An ideal system would have a return in every room where there is a supply vent, but ideals are rarely achieved; there usually are fewer returns. Return ducts are in a floor or at floor level on an inside wall. Supply ducts are on outside walls. Measure the size of return openings with a tape measure. Count and measure the supply ducts. That should give you a difference between supply and return duct capacity.
- 2). Modify an existing return opening vertically; a 14-by-20-inch return can be expanded to 24, 28 or 30 inches high. Remove the metal air filter frame and cut wallboard to the desired height by cutting it with a utility knife and peeling off with a crow bar. Replace the flexduct from the return opening to the unit with the larger size. Seal seams with heat-resistant tape. Install new framing of the right size to hold the air filter by screwing it to the studs on each side..
- 3). Enlarge the return horizontally if a vertical expansion is not possible. Determine a size; return frames and filters come in dimensions up to 30 inches, but 28 inches will fit a space between wall studs. Mark the wall where the top of the new frame will go. Remove the wallboard at that point, to reveal three studs. Cut the center stud with a reciprocal saw at the top of the opening. Cut a 2-by-4-inch header and screw it to the two outside studs. Cut and install short 2-by-4-inch studs to go from the header te the wall base plate. Remove old ductwork and install new duct of the proper size, sealing seams with heat-resistant tape. Screw a new filter frame to the studs on each side.
Source...