HVAC Duct System Installation Procedures

Find the maximum airflow from the unit, from a label on the unit or the owner's manual. Determine the distance from the unit to the longest vent outlet, based on a trunk and branch system with a large central supply duct and forks off to vents. Get the distance of each branch from the main duct to the vent. Pick a type of duct--flexible ductwork, or flexduct, is the most common.

Use that basic information in an equation based on air velocity, loss of flow due to friction and other factors, or enter the data into an online program, like The Engineering ToolBox (see Resources), to calculate the size of the central supply duct and branches. Figure a return system, too, from the air filter entry point, usually in a hallway or other central location, back to the unit. Main supply ducts are usually 12 inches or so and branches about 6 inches.

Look for routes through the basement or attic to run supply and return ducts. Make duct routes as straight as possible, with no tight bends that will crimp flexduct. Determine a route to the attic, if the house has no basement or access under the floor, usually through a closet or similar opening. Make a map of the routes on the house layout.

Mark water heaters, hot water pipes, electrical boxes and any heat source or other impediments to ductwork. Look for support points in a basement or under a floor--places where ducts can be fastened with metal duct hangers, straps around the flexduct that fasten to joists or other supports on either side. Use natural supports, like gas pipes or floor braces, where possible. Lay flexduct across ceiling joists in an attic.

Begin installation at the unit. Run the central duct first, securing it where necessary; it will need to be fastened to wall studs for a vertical run to an attic. Mark spots for branch connections. Cut flexduct with tin snips and install metal sleeves to fasten smaller branches to the main supply. Secure connections with adjustable straps to bind the flexduct to the sleeves and seal the seams with heat-resistant tape.

Fasten the branch ducts to the vent openings with adjustable straps and tape; vents have circular openings to accommodate flexduct. Secure the main supply duct to the large supply plenum on the unit once all branches are installed. Install returns, typically much larger ducts, to the opening in the air filter entry point and to the return plenum on the unit; return runs typically are much shorter than supply runs.

Force the system blower to turn on once all ducts are installed and check for leaks. Look for flapping flexduct and listen for whistles that indicate air is escaping. Seal any leaks with tape.