How to Install Flex Ductwork

Design a flexduct installation with a "trunk and branch" system, using a large central supply duct with smaller branches to individual vents along outside walls. Include a return system, to collect air from inside the house and take it back to the heating/cooling unit. The airflow for supply and return systems must be balanced, to collect all the air being emitted by the unit. Size flexduct to the airflow from the unit according to the manufacturer.

Install flexduct in a basement or a crawl space using floor joists for support. Fit metal strap hangers around the flexduct and screw them with a screw gun to wood joists on either side. Run flexduct as straight as possible and avoid any sharp bends or places that might crimp the flexduct and impede airflow. Use as many natural supports like drain pipes and gas pipes as possible. Hang flexduct solidly but with enough "play" or sag that it is not stretched.

Put flexduct in an attic if no under-floor space is available. Find a closet or special wall space large enough to accommodate both supply and return ducts -- probably two spaces, one for supply and one for return. Get to the attic as close as possible to the heating/cooling unit location and run flexduct over or between attic joists. Secure it in a few places to prevent it from blowing in any wind that gets in. Run smaller supply ducts to ceiling vents or down wall cavities for wall vents.

Try to place return collection points in the same space with supply vents or in a central hallway or other area where air will flow naturally. Put all supply vents on exterior walls, preferably near windows, and all return points on inside walls. Fasten filter frames to studs on the sides of the return opening, sized to the size of the return duct feeding them. Use standard filter sizes to make replacement easy.

Join sections of flexduct with metal connectors, which fit inside the ducts and are sealed with heat-resistant tape recommended by the manufacturer. Use T or Y connectors to join central supply and individual vent ducts. Use Y connectors when possible so only a single opening has to be made in the central supply duct to supply two vent ducts. Seal all seams thoroughly with tape. Cut flexduct with tin snips to make connections.