Sketch the layout of the ductwork that needs to be replaced, making notes of branch lines and transitions. Inspect as much of the ductwork as possible, even though it may be in the crawl space, basement, or attic. This information is needed not only to purchase the replacement pieces but also to comply with local codes. These codes ensure that the new duct system will have the right airflow, be of the right gauge of metal and are the right distance from combustibles.
Measure the size of the existing ductwork and transitions, as well as the length of each run of ductwork. If the ducts are covered in insulation, cut it back to get an accurate measurement. Be sure to tape the insulation back in place so it works as efficiently as possible before replacement. With all the dimensions and lengths known, it is time to purchase the replacement ductwork.
Remove the old ductwork once all the new pieces are on site. Use the utility knife to cut away insulation and to expose screws that may be covered with tape. Use a drill equipped with the nutdriver attachment to disassemble the ductwork. Where necessary, use the flat head screwdriver and a hammer to pry duct sections apart. Replace the ductwork in sections, rather than completely removing the old ductwork and then installing the new ductwork. Doing it this way helps prevent the problems that could occur if all the old ducts were removed at one time.
Seal all the duct joints with the foil tape. Make sure that the tape is sealed well and that there are no air bubbles or excessive wrinkles in the tape. Only use a foil tape for this job. While duct tape has numerous uses, sealing ducts is not one of them and will fail over time.
Replace duct insulation and strapping, as needed. Since the effort is being made to upgrade the old ductwork, go ahead and use new insulation, as well. The strapping is usually sold in rolls and can be cut with metal snips to the desired length. Use tape to hold the insulation together and new screws to hold the strapping in place. Continue replacing duct, insulation and strapping until the job is complete.