How to Size and Install Flexible Duct
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How to Size and Install Flexible Duct

How to properly size and install flexible duct for your HVAC system to maintain the design performance of the system.

Flexible duct has many advantages in the heating, ventilation, air conditioning (HVAC) arena. It is easy for the novice to use and it also reduces installation time compared to sheet metal duct. But you need to be careful when using when attempting to use it as a direct replacement for smooth, galvanized duct as they have very different performance characteristics.

Flexible duct has a unique design that increases airflow friction loss compared to the same size smooth-walled galvanized duct. To maintain an equivalent performance to hard duct requires a larger diameter flex duct. Proper sizing of branch lines will increase the efficiency of your air handling unit and improve comfort and indoor air quality.

Friction loss in straight duct is dependent on the relationships of duct diameter, air velocity in the duct, and duct roughness as major components. Flex duct with its helical corrugated construction is going to be much rougher than sheet metal duct. The roughening effect is increased when the flex duct is not stretched out to the extent possible during installation. Slack in the duct allows the coils of reinforcing wire to relax, which bunches up the polyester and pushes it into the interior of the core, adding more resistance to airflow.

Sizing Charts

Sizing charts and calculators for duct sizing are available from many sources and a few online calculators are included at the end of this article. By spending a few minutes you can easily see the differences between the friction losses for galvanized metal verses flexible duct. It is worth noting that for a fixed duct diameter, as the velocity in the duct increases, the friction loss increases twice as fast. This means that if the velocity in the duct were to double, the friction loss would be four times greater. A useful rule of thumb that is very reliable is to increase the size of flex duct one diameter to neutralize the added friction loss compared to that of galvanized duct for the same CFM (cubic feet per minute).

Account for bends and offsets in the flexible duct. A 90-degree bend has pressure drop equal to approximately twenty (20) lineal feet of flexible duct. So each 90-degree bend will add twenty (20) equivalent feet to the length used for sizing calculations. A gradual 45-degree bend has a pressure drop equal to about ten (10) lineal feet of flexible duct. A 180-degree offset has a pressure drop equal to about forty (40) lineal feet of flexible duct.

There are a few variables that you may not be familiar with when it comes to HVAC duct design, most notably air velocity and friction loss. For residential applications branch lines are sized at 600 fpm. Friction loss is typically set at 0.10 inches of water. The last variable is duct length which refers to the equivalent duct length that accounts for bends and fittings that is discussed above.

Equivalent Duct Length for a 45 degree bend


A further penalty in performance will occur if flexible duct is compressed from its round shape to an oval shape, which often occurs when the flex duct is squeezed it into a joist space. Manufacturers allow for up to a 20% reduction in diameter only if it occurs in one spot, but not over any distance or repeatedly. This means that a flexible duct maybe be slightly compressed at one point without severely affecting the performance of the duct, such as a wire that is pulled across the duct. The friction loss for flex squeezed into an elliptical shape over any distance is large and the loss of airflow will be significant.

Cubic feet per minute airflow rate still equals the air velocity times the area of the duct in which the air is flowing. Increasing the area of the duct will slow the velocity of the air and reduce pressure loss. Keep in mind that the long-term system performance will be affected by the one-time cost of the flex duct. Increasing flex duct one size to offset its higher pressure loss compared to smooth duct is highly recommended.

Many installers and engineers will not use flexible duct on return lines due to the fact that the negative pressure can compress the walls of the duct and increase friction. Manufacturers design flexible duct to withstand slight negative pressure, but real-world installation of the flex duct makes it impractical.

Codes and Standards

Several codes apply to the installations requirements for flexible duct, especially NFPA 90A or 90B.

Some other important restrictions for flex duct to make note of are as follows:

Shall not be used to vent appliances for cooking, heating and clothes drying unless approved and recommended by the appliance manufacturer.

Use the minimum length of flexible duct to make connections. Excess length of flexible duct shall not be installed to allow for possible future relocations of air terminal devices.

Flexible duct installation


Allow a minimum of 1 duct diameter bend radius


Air Diffusion Council

Online duct Calculators

Hart & Cooley



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Comments (1)

Good work Daniel.