How to Add a Home Heating Duct

Home additions usually create the need for a new heating duct or two.

Remodeling often involves ductwork.Remodeling often involves ductwork.
Adding a new duct to an existing heating system isn't for the faint of heart, but if you're comfortable doing major renovations, adding a duct isn't an unreasonable task. Six-inch round duct ("pipe") is the norm for home heating, and is suitable for most uses. Air conditioning systems have greater requirements, and may need larger pipe. If you are unsure, consult a HVAC professional for duct size requirements. Supplies and tools for this project are available at most well-stocked home centers. Expect to spend as much as four hours installing your new duct.

Locate an existing trunk line. This is the large rectangular duct that originates at the top of the furnace, and has smaller round ducts exiting it. This is a good time to decide where to locate the register; the best locations are near sources of heat loss, like near windows.

Cut a 6-inch-diameter hole in the side of the trunk line. Use the starting collar as a template and draw a circle on the duct, using the felt-tipped marker. Make an access hole with the hole saw, then cut to your marked line, using the tin snips.

Attach the starting collar to the duct by inserting the tabs into the duct, and then bend the tabs over inside the duct.

Slide the female end of a 6-inch round duct over the starting collar. Add as many sections as necessary to connect to the register boot. If you need to cut a section, cut the female end. Secure all connections with three self-drilling zip screws, and seal with foil tape.

Install hanger strap to support the new ductwork. Screw the hanger strap into a floor joist, run it down below the new duct, and back up to the joist, forming a U; secure the hanger strap to the joists and to the duct, using zip screws.

Cut a rectangular hole in the floor for the register boot. Using the boot as a guide, mark the hole between floor joists. Drill ¾-inch holes in the corners of the rectangle, and cut out using the reciprocating saw.

Attach the boot to the floor with zip screws; screw through the inside edge of the boot into the edge of the subfloor.

Things You Will Need

  • Measuring tape
  • Permanent felt-tipped marker
  • Cordless power drill
  • Drill bits
  • Tin snips, offset style
  • Starting collar (6-inch)
  • 6-inch round duct
  • ½-inch hex head "zip" screws
  • Foil tape
  • 6-inch register boot, 90-degree
  • Reciprocating saw


  • Temporarily installing the register boot makes it easy to measure for the last piece of pipe in a duct run; just measure the gap between the boot and the penultimate pipe, and add three inches.
  • If your ductwork is in an uninsulated crawl space or attic, insulate the duct.
  • Use foil tape, not "duct tape," to seal ductwork.
  • Plan your duct run carefully, keeping it as straight and short as possible.


  • Sheet metal edges are extremely sharp. Use extreme caution and wear gloves when working with sheet metal.
  • Wear safety glasses or goggles when working with power tools.
  • Have a first aid kit available on the job site.

About the Author

David Brown began his writing career while still in college, writing and editing research grants and scientific papers. His work has appeared in such journals as "The Journal of Clinical Investigation" and "Gastroenterology." He currently owns a construction company in Boulder, Colo.