A return air duct is what carries air back to the furnace after it is dispensed throughout the home through the supply duct. Without a return air duct, the air would not properly circulate and your energy costs would rise. Return air ducts usually use ceiling joist spaces as the actual "duct," which then runs back toward the furnace. These joist spaces empty into an actual duct that runs perpendicular to how the joists run, and then down into the side of the furnace. With the right tools, you can install a return air duct successfully.
Locate where you want your return air ducts. Cut a hole low in the drywall where you want to put your return air grille. Size the hole appropriately to the grille size. Through the hole, cut through the floor. This should give you a hole into the basement ceiling joist space. Continue to repeat the step until you have the amount of return air holes you desire. You should have at least one per room except for the bathroom.
Go into the basement and locate the holes. Work from the farthest hole back and install return air panning back toward where the furnace is. Nail the panning to the bottom of the ceiling joists. At the farthest hole, notch the panning to fit, bend it up, and attach it to the floor to close off the space. By the furnace, stop all the panning in the joists at the same spot. Notch this side also and take it up to the floor. In a line perpendicular to the joists, cut holes in the panning. This will be how the air gets from the panning into the return air duct that will run to the furnace.
Install the square return air duct perpendicular to the joists. You want to run it so it covers up all the holes you cut in the panning. Cut holes in the top of the return air duct to match the holes in the panning. This allows the air to get into the return air duct. Return air duct is connected together using s-slips and drive cleats. (See reference 2 for pictures of s-slips and drive cleats.) The return air duct should be installed tight to the bottom of the ceiling joists so it makes a tight seal with the panning. Secure the duct to the ceiling joists with 1-inch galvanized hanger strap.
Cut a hole in the bottom of the return duct next to where the furnace is. This is where you'll attach another duct that will run down and in to the furnace. Cut a hole in the side of the furnace that matches the size of your filter rack. Make the connections from the bottom of your return cut to your filter rack and attach to the furnace. Cut duct as necessary to fit.
Install your HVAC filter into your filter rack. Caulk all joints and seams of the duct with a high-quality clear silicone caulk for best energy performance.
Screw all your return air registers over the holes that you cut upstairs. Your return air duct system is now complete.