How to Determine Friction Rate of a Flex Duct

The friction rate of sheet metal ducts is standardized at 0.1 inches of water column per 100 feet of duct. Because different manufactures of flexible ducts make their products in different ways, there is no standard for flexible ducts. You can use the manufacturer's air friction loss data from the duct packaging to determine the friction rate of a duct when fully extended. The design inside flexible ducts impedes airflow over long distance runs, and this is amplified when the duct is compressed. Not fully extending the flexible duct changes the air friction dramatically. In fact, the friction rate increases in proportion with the compression of the duct.

Things You'll Need

• Measuring tape
• Calculator

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Instructions

1. • 1

     Determine the percentage of compression in the duct. Measure the length of the duct fully extended, then measure the distance between the two connections.

   • 2

     Divide the length of the run by the length of the duct. Turn this number into a percentage by multiplying by 100. Four percent or lower is considered fully extended.

   • 3

     Decide how much this percentage of compression causes a higher friction rate. A percentage between 15 and 30 percent doubles the friction rate. Percentages between 30 and 60 percent can increase four times over the standard set by the fully extended duct.

   • 4

     Multiply the manufacturer's friction rate when fully extended by the increase indicated by the compression. This is the possible air friction rate of your duct.