How to Replace HVAC Ducts


Usually heating, ventilating and air-conditioning (HVAC) ducts last a very long time. Made of galvanized metal, they can last almost a lifetime before corroding and deteriorating. Sometimes though, for unknown reasons, a duct can fail early. When this happens, the duct has to be removed and replaced. There are also circumstances when an HVAC duct must be replaced because it is in the way of something else that needs to be installed. This happens frequently during home remodels. With the right tools, replacing HVAC ducts can accomplished fairly easily.

Things You'll Need

  • Cordless drill
  • 8 sheet metal screws, 1/2-inch
  • 1/4-inch nut driver bit
  • Screwdriver
  • Hammer
  • Vice grips
  • Pipe crimper
  • Aviation snips
  • Hand seamers
  • Various HVAC duct fittings
  • S-slip connectors
  • Drive cleats
  • Safety glasses
  • Work gloves


  • Verify what kind of HVAC duct you will be replacing. Round ducts and square ducts require different methods to replace. Round ducts are screwed together. Square ducts are joined with S-slips (they look like an "S" if you look at them from their edge) and drive cleats (these lock the two ducts together). If you have round ducts, read on. If you have square ducts, jump to step 5.

  • For round duct, unscrew the screws from the duct. Older ducts might be fastened with flat-head screws. Newer ducts have screws with the 1/4-inch hex heads. You may have to unscrew multiple joints to get it all apart. Undoing a connection at an elbow helps with taking things apart.

  • Make out a list of required round duct and fittings, depending on your needs. Head to your local home improvement store and purchase them.

  • Reassemble your round HVAC duct. Screw all joints together with your sheet metal screws.

  • If you have square duct, disconnect it. Bend over the drive cleat with your vice grips and hammer it off. Repeat this on the other side. You may need to break all connections back to an elbow. This process will make it easier to take all the connections apart.

  • Make out a list of required duct and purchase it at your local home improvement store. Some specialty fittings might need to be fabricated at a local HVAC shop.

  • Reconnect your square duct. Cut the S-slips 1/2 inch smaller than the duct. Cut the drive cleats 2 inches longer than the side they will go on. The S-slips go on the raw edge; the drive cleats go on the edge that's bent over. Put the S-slips on one duct and slide it into the other duct. Hammer up the drive cleat, ensuring its locking the two ducts together as it moves up. The drive cleat should stick past the duct 1 inch on each end. Pound these over so they lie on top of the S-slip. Continue these procedures until all the duct is back together.

Tips & Warnings

  • If you need to cut a joint of round duct down, use your aviation snips to cut it. Crimp the end with your pipe crimper so that it will slide into another piece of duct.
  • If you need to cut a joint of square duct down, you will need aviation snips and hand seamers. The hand seamers will allow you to make the edge of the cut joint suitable for connection.
  • Caulk all seams and connections to improve the efficiency of the HVAC unit.
  • Consult with an HVAC technician if you are unsure about how to proceed.
  • Wear safety glasses and work gloves when working with HVAC ducts. They are extremely sharp. They can cause serious lacerations and eye damage.

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