Inspect the attic for obstructions, such as hot water pipes or electrical boxes or fixtures. Avoid anything that might produce heat or interfere with flexible duct, which is formed with coiled wires, fiberglass insulation and a metallic outer coating. Locate vents. Place them in ceilings for easiest access if no vents are already installed.
Find the most direct route from the heating/cooling unit to the attic. Units usually are located just outside house walls. Pick an installation site near a closet or other good access point to the attic if you can. Examine the house for large closets with unused space, or walls that could be easily enlarged to accommodate basic supply ducts, which are usually 12 to 16 inches in diameter..
Draw a map of the attic to route a main supply duct from the attic entry point to the farthest point of the house, preferably along the attic's center, which normally forms the largest open space in an attic. Identify all points for branches to room vents and make sure there is enough room for the 6-inch ducts and space to work.
Lay the flexduct in place, running it over attic insulation. Place it inside attic joists if possible, and across them when necessary to reach outlets. Extend flexduct fully, but don't stretch it. Leave it loose enough to move slightly. Avoid any sharp bends or other changes that might crimp or compress it. Secure it about every 6 feet with metal straps across the duct, fastened with nails and a hammer to attic joists.
Fasten supply ducts to vent bases — typically round to conform to the duct — with adjustable strap fasteners. Connect small ducts to the central supply with metal sleeves, which fit inside both duct ends and are secured with straps. Seal all joints with heat-resistant tape. Secure the basic supply duct to the output opening on the unit with straps and tape.
Install a return system, to collect air from inside the house and return it to the unit. Locate the return entry in a central location — like a hallway — on or near the floor. Run the return duct directly to the unit if possible, or run it up a wall or closet, across the attic joists and back down to the unit. Use the same attic access point for supply and return ducts to simplify installation.