How to Run Ductwork

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If your area permits it, installing flex duct yourself will save on your HVAC.
If your area permits it, installing flex duct yourself will save on your HVAC. (Image: air conditioner, conditioning image by Greg Pickens from <a href='http://www.fotolia.com'>Fotolia.com</a>)

Running duct work and all other heating and air conditioning-related jobs are regulated by the local building codes and the HVAC industry. Building codes are as varied as the state and county agencies who write them, so be sure to check with your local building inspector’s office before doing duct work yourself, only to learn it all has to be torn out and replaced by a licensed installer. Flexible duct has made this a doable home owner task, at least mechanically, if you do not mind crawling around under the house or in the attic all day.

Things You'll Need

  • Measuring tape
  • Marking pencil
  • Reciprocating saw
  • Vent boots
  • Knee pads
  • Wire cutters
  • Metal, beaded joint connectors
  • Aluminum tape

Locate, measure and mark all interior floor vent locations throughout the house using a vent template to outline the vents on the sub floor. Be sure to avoid floor joists, and move the vent location slightly if joists are found within the marked areas. Put at least one vent in each room, and more if the room is larger than 10 by 12 feet.

Use a reciprocating saw (saws-all) to cut out the Step 1 vents you located and outlined. Cut the inside edges of your marks to get a tight vent cover fit. Go back, drop in and attach the vent boots after cutting out the vents. Now you have the piece in place to attach the flex duct to when you run it from the supply and return.

Try to place your vents near walls instead of the middle of the room.
Try to place your vents near walls instead of the middle of the room. (Image: House interior design image by Aleksandar Radovanovic from <a href='http://www.fotolia.com'>Fotolia.com</a>)

Calculate, cut and install the flex duct for the already installed heat pump/AC unit. Use an industry manual (Air Conditioning Contractors of America Association, Inc. chart or other local building codes) to calculate supply side size based on the greater of the heat or cooling load for each room. Determine vent size by the amount of air needed per room, the length of the duct, its number of turns and the static blower capability.

Use the trunk and limb system to install the flex duct. Hang the duct using wire or saddle material so it is not restricting air flow. Attach the hangers to the floor joists at five-foot intervals.

The trunk is the supply and return, and the limbs are the ducts run to each room.
The trunk is the supply and return, and the limbs are the ducts run to each room. (Image: trunk image by Dave from <a href='http://www.fotolia.com'>Fotolia.com</a>)

Measure the farthest vent distance from the supply side off the plenum. Limit this distance to 25 feet or less to get maximum air delivery. Use a pair of wire cutters and cut the piece of flex duct to length after peeling back the duct jacket and insulation. Attach the duct to the supply using a metal, beaded sleeve, zip ties and aluminum tape. Pull the flex duct over the bead of the attachment sleeve and strap into place with a zip tie. Secure the connection with aluminum tape.

Pull the flex duct, fully extending it, to the boot you dropped into the vent hole in Step 2. Attach this end to the vent boot just like you attached the other end to the supply. Do not leave slack in the flex duct, since this will cut down seriously on air flow effectiveness. Use metal elbows and straight pieces to join these runs as you encounter sharp turns. You now have run your first flex duct.

Tips & Warnings

  • Be sure to have the central unit installed by a licensed AC technician.
  • Do not bend or crimp the flex duct around objects; this will restrict air flow.

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