Remove existing ductwork where necessary. Run the new flexible air duct under the house from the floor, go back to the bottom of the air handler and attach it to the metal vent box or plenum (Image 1).
Note: If the air duct is not long enough, it is possible to add additional length. Attach a second air duct to the first by stripping the insulation and tucking it into the first section approximately 6"-10" and seal with tape. Fold the insulation back into place and roll over the outer layer (Image 2).
Use zip ties, placed every few feet, to attach the entire length of the duct to the bottom of the floor joists; This keeps the conduit up and off the ground.
Seal the new flexible conduit to the new box with zip ties, keeping the connection between the conduit and the box as tight as possible. Once everything is in place, attach the end of the ties, seal everything with foil tape, and burnish with a plastic rib.
To hold the new vent in place, it may be necessary to create a wooden frame or casing. Measure the opening and cut the 2x4s to size. Nail the frame into the opening and snap the new vent into place (Image 1). Center, using 2" x 4" wrenches and screw into place (Image 2).
Fill the spaces between the floor and the new frame with an insulated duct board. Cut the board to size, making sure it slides and fits snugly on both sides of the frame (Image 1).
If necessary, further seal the vent with adhesive mastic cement, which is painted, making sure there is no air leakage around the vent (Image 2).
