How to Add a Return Vent and Duct to Furnace

Add a return for a furnace either as close as possible to the unit or in the same area where new supply outlets were placed. Locate returns if possible in a central area where air will flow naturally, such as a hallway. Place returns, which include air filters, for heating or furnace units near the floor, where the cold air will collect, in a space where ducts can be run easily to the furnace.

Determine a routing for flexible ductwork from the return to the furnace. Run ducting through a basement or crawl space, if possible, or through a ceiling or attic if no other space is available. Find a route to fit the size of the return duct, which can vary from 6 to 12 inches; smaller ducts normally will suffice for an added return if a main return is already in place.

Locate wall studs in the selected return area using a stud finder. Remove drywall with a pry bar to expose the studs. Put returns between studs to avoid having to cut studs and add bracing, which can be complex. Size return openings for a frame with a 14-inch air filter between 16-inch spaced studs. Adjust the height to get the needed volume; a 14-by-20-inch filter will provide about 300 square inches of return, while a 14-by-24 will afford 336 square inches; base the size of the return on the opening needed to collect return air in balance with the furnace output.

Install the metal frame for the return with screws through installation holes in the side, driven with a screw gun into the studs. Set the frame in place before adding any ductwork. Fasten the duct to the metal collar on the return frame with an adjustable strap and seal that seam with heat-resistant tape. Run the duct through the basement or up a wall and through the top, securing it about every 5 feet with a metal strap over the duct fastened to studs or joists.

Secure the duct to the return plenum; you may have to remove an existing return duct and add a Y-connecting collar to the plenum or install a Y-connector to tie the new return into an existing return. Make connections with metal sleeves that fit into the duct and the plenum. Fasten metal collars to metal plenums with sheet metal screws and connectors to ducts with straps. Seal all seams with heat-resistant tape.