How to Fix a Hole in Your Furnace Duct

Fixing Small Holes

• 1

  Shut off the power to the furnace. Clean the area around the hole -- up to 1/4 inch in size -- with a clean cloth. Wipe off dirt, debris and oil, or the mastic won't adhere to it properly.

• 2

  Apply a liberal amount of mastic to the hole. You'll want to end up with a thick covering when you're finished. Use a brush, trowel or a caulking gun. Wear rubber gloves if you use your hands to apply it.

• 3

  Press the mastic into the hole. Spread it out evenly so that it's at least 1 inch beyond the hole on all sides.

Fixing Large Holes

• 4

  Shut off the furnace. Clean the area around the hole -- over 1/4 inch in size -- with a clean cloth. Wipe off dirt, debris and oil.

• 5

  Apply a generous coating of mastic to the hole. Be sure to spread the mastic at least 1 inch past the edges of the hole.

• 6

  Cut enough strips of mesh tape with a sharp pair of scissors to cover the hole in the furnace duct. The strips should be approximately 4 inches long.

• 7

  Place the tape strips over the hole and press them into the mastic. Overlap the edges of the strips. The tape will add reinforcement to the repair.

• 8

  Apply at least one more coating of mastic over the tape so that it's completely covered.