How to Install Heating Ductwork in the Basement

Calculate the size of all ductwork needed -- both supply ducts from the heating unit to the vents and a return system to collect "used" air and take it back to the unit. Use an online calculator like The Engineering Toolbox or Elite Software (see Resources) to make these complex calculations, based on airflow from the unit, length of ducts and other factors.

Make a map of the basement, marking water heaters, hot water pipes, electrical boxes and other elements to be avoided. Keep ducts 12 inches or farther away from any heat source. Locate all the vents or outlets in the floor above and the return entry points. Identify natural supports, like braces between floor joists, gas pipes or other objects with enough space to hold flexible ducts -- typically 16 or 18 inches wide for main ducts and 6 or 8 inches for vent ducts.

Route a main supply duct from the heating unit to the far end of the house, usually down the center of the house, with branches to feed each individual vent. Make duct lines as straight as possible; avoid sharp bends that crimp the duct. Include return ducts from the air filter entry point or points to the unit. Remember that return ducts will usually be larger than the main supply ducts.

Install the main supply duct first. Run it between floor joists as much as possible, over gas pipes or other natural supports. Put thin metal hanger straps, typically about 2 inches wide, around the duct and fasten them with nails and a hammer to joists on either side of the duct. Don't crimp the duct; let it sag between supports but keep it supported about every 6 feet.

Run branch ducts to vents between floor joists or across those elements, depending on how the floor framing is laid. Secure these ducts to joists about every 6 feet. Place branch ends over the vent openings and secure them with mounting straps and heat-resistant tape. Connect the branches to the main duct once all branches are installed with metal sleeves, which fit inside the ducts and are fastened with adjustable straps and heat-resistant tape.

Complete the system by connecting the return ducts to the frame holding the air filter; ducts typically slide over a circular return opening in the unit and are secured with adjustable straps and heat-resistant tape. Run return ducts as straight as possible back to the unit. Put these between joists when possible and fasten with straps across the duct, nailed to joists on both sides. Seal all seams on both supply and return sides with heat-resistant tape so no air can escape.