How to Identify HVAC Sheet Metal

Determine the Type of Metal

• 1

  Cut a 3-inch square on the surface of any insulation installed on the ductwork with a sharp knife. Remove the insulation to expose the metal.

• 2

  Examine the exposed section of metal. The metal is galvanized steel if you notice a series of crystal structures on its surface. Place a magnet on the exposed metal. If the magnet sticks, it confirms the metal is galvanized steel. A uniformly colored metal that is not magnetic is either stainless steel or aluminum.

• 3

  Determine the hardness of the nonmagnetic metal by scratching the surface of the metal with the tip of a sharp knife blade. Rub the tip of a finger over the scratch. The knife will create a shallow, almost invisible scratch on stainless steel or a deep gouge on the surface of aluminum.

Determine the Metal Thickness

• 4

  Slide the tip of a flathead screwdriver under the edge of a longitudinal duct seam. Hit the handle of the screwdriver lightly with a hammer. Pull the screwdriver handle away from the duct to lift the edge of the duct.

• 5

  Push a slot of a sheet metal gauge over the lifted edge. Move to another gauge slot if the first slot does not fit tight on the edge. Continue moving until you reach a slot that fits snugly on the edge. Read the decimal numbers printed on the side of the sheet metal gauge.

• 6

  Find the decimal numbers on a sheet metal thickness chart. Follow the line to the gauge equivalent of the decimal.

• 7

  Combine the metal gauge with its determined type to identify the HVAC metal. For example, a measured thickness of .1196 on a metal that shows crystalline structures on its surface would mean the HVAC duct is made of 11ga galvanized steel. Another example, a measured thickness of .032 of a solid surface, non-magnet metal that scratches easily would mean the duct is made of 20ga aluminum.