A/C Ductwork Installation

Measure distances to be covered by ductwork, using a tape measure or a house plan. Design a "trunk and branch" routing, with a large main duct with extensions to the sides to reach all vents. Plan a return system, to carry "used" air back to the central unit. Supply and return systems must be balanced; the return must have the same air flow as that coming from the unit. Determine sizes for ducts -- main ducts will be larger than branches -- and buy the materials needed to cover the runs.

Avoid any sharp turns or bends and any obstacles like electrical boxes and fixtures or hot water heaters or pipes. Run ducts through a basement or crawl space if possible, through an attic or ceiling, if necessary. Put ducts going up inside wall cavities; make sure they are big enough to accommodate the flex duct without pinching it. In a house under construction, a large wall can be built to handle this.

Support flex duct about every 6 feet, either with strap metal hangers that go around the duct and fasten on either side with a hammer and nails or with built-in supports like braces across joists. In attics, lay ductwork over joists and fasten it in a few places to keep it from moving with any wind that blows in. In ceilings, install between joists supported by hangers nailed between joists.

Secure all seams, where two sections meet, either for longer runs or for branches. Use metal connectors, which insert into ducts on each side or end. Seal seams with heat-resistant tape. Fasten the supply and return systems to the entry and exit points on the unit, metal tubes which come through the wall into the house. Secure these with sheet metal screws and tape.