How to Cut an HVAC Duct

HVAC ducts consist of a variety of materials.

HVAC ducts run from the main unit to the home's air vents and come in two forms: a rigid steel cylinder and a flexible cylinder. Both forms have their uses, and often a single HVAC installation uses both types of ducts. Because these ducts come preformed to certain lengths, cutting them is required to fit your specific installation area. Cutting the ducts requires different tools, and rigid metal ducts require more time to cut.

Things You'll Need

• Tape measure
• Permanent marker
• Drill
• 3/8-inch drill bit
• Protective glasses
• Aviation snips
• Utility knife
• Hacksaw with metal cutting blade (Optional)

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Instructions

Cutting Rigid Ducts

1. Draw a line on the metal duct where it needs to be cut. If you are installing a straight piece of duct work, measure the installation area with a tape measure then transfer the measurement to the duct. If you are attaching one duct to another, place one section on a flat surface and hold the second one up to it where they will be attached. Trace around the edge of the second section to create the cutout line that will allow the second duct to slide inside the first duct.

2. Install a 3/8-inch bit into a drill and put on protective glasses. Place the tip of the drill bit on any area of the marked line and squeeze the trigger to drill through it. Remove the bit and drill three to four additional holes along the line in the same manner. It does not matter how far apart the holes are since you are simply creating openings to assist with the manual cutting process.

3. Set the drill aside and pick up a pair of aviation snips. Open the snips, just as you would a pair of scissors, and insert the curved lower blade into one of the holes. Cut around the marker line with the snips until you reach the starting location.

4. Detach the two duct pieces from each other if you are cutting them in half. If you are cutting out a hole, reach inside the duct and remove the metal piece.

Cutting Flexible Ducts

1. Measure the installation area where you are installing the flexible duct work to determine the proper length.

2. Lay the duct work on a work table and straighten it out so there are no curves in it. Measure from one end of the duct to the cutting location with a tape measure. Draw a line all the way around the duct with a marker.

3. Cut around the marker line with a sharp utility knife, puncturing through both the outer foil membrane and inner insulation. If you meet resistance from wires, simply cut around them as much as possible.

4. Set the utility knife aside and pick up a pair of aviation snips. Cut through each of the wires that run through the cut line until the flexible duct separates.

Tips & Warnings

• Always cut the non-crimped end of rigid metal duct work.

• You can also cut rigid metal ducts with a hacksaw equipped with a metal cutting blade.

References

• Hammer Zone: Adding A New Duct To A Heating Or Cooling System
• Alpine Home Air: Ductwork (PDF)
• Weldin Engineering Company: Flexible Duct -- Boon or Bane?