- 1). Plan and measure your ductwork. Make a rough diagram of your house and mark the routes for ductwork from the heating/cooling unit to each register and room. Then measure those distances; you don't have to be precise, but it will tell you how much material to buy. Check with your heating/cooling manufacturer or installer about what size ductwork will be needed to carry the airflow. In your plan, note all obstructions -- water pipes, electrical wires, flues or chimneys. Plan your ductwork to avoid those and any sharp turns or places where flexduct would be cramped or crimped.
- 2). Run your ductwork and leave it loose until it you have covered all the distances and angles. Then attach it between floor joists, either nailing boards across the joists or using metal strap hangers which fit around the flexduct and are nailed to joists. You need support every five feet. For vertical runs, from a basement to an attic or from one floor to another, mount supports every 6 feet. You'll have to cut holes in flooring if you're going between levels in an existing house; in new construction, you can run ductwork before flooring is installed.
- 3). Stretch the ductwork to its fullest, so it is not bunched up any place. Seal all seams and joints, using metal collars at least four inches long between sections, covered with heat-resistant tape. You also can clamp the inner core. Some experts suggest duct tape, others prefer other tapes because duct tape can loose its adhesion with time and heat. Install fireblocking anywhere flexduct goes through a floor or ceiling; fiberglass insulation usually will suffice. Make sure there are no gaps or holes where air can escape
- 4). Complete the job by attaching supply and return ductwork to appropriate collars on the heating/cooling unit. The supply ducts distribute hot or cool air; returns collect "used" air back into the unit and are normally located close to it and behind filters. Test your ductwork by forcing the unit blower to come on and look for blowing insulation or places were air can be detected coming out. Repair any gaps or tears.
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