DIY: Flex Ducts

Draw a diagram of the house, showing every floor. Mark any obstructions, like walls, exhaust vents, electrical boxes, water heaters. Lay out duct routes with a large central supply duct that forks off to individual areas; place vents against outside walls. Plan a return route, with return openings on interior walls, preferably near a floor, although some may be in an attic.

Size the supply trunk to the output from the unit and run it as straight as possible through the house. Make any openings for ducts that must go up, from a basement or to a second floor. Put these inside walls if possible. In new construction an extra-large wall space can be created to accommodate these; in an existing house run vertical ducts through a closet corner or similar space. Put supply branches inside basement or ceiling joists or an attic if necessary.

Install supply ducts, starting from the unit and moving through the house. Put these between joists in a basement or ceiling, securing with metal hanger straps nailed to the joists across the flexduct. Support flexduct about every 6 feet. Use an attic if necessary, laying ducts across joists and fastening to keep it from blowing. Cut ducts with tin snips as needed. Connect pieces of duct with metal sleeves, which fit inside the two pieces of duct; use heat-resistant tape to make joints and seams air tight.

Place the return ducts and connect both supply and return ducts to the unit; it will have sheet metal pipes extending into the house from the unit. Secure these with heat-resistant tape. Test the system for leaks by turning on the blower. Some units have a switch to activate the blower; on others, adjust the thermostat up or down until the blower starts. Look for any duct flaps blowing or air flowing out and seal with heat-resistant tape.