Flex Duct Derating: How Length and Bends Cut Your CFM
Flexible ductwork reduces airflow capacity compared to equivalent rigid metal ducts. This reduction—called derating—occurs because of friction loss in flex's textured interior, compression from improper installation, and energy loss at bends. Understanding these derating factors is essential for sizing ducts correctly and ensuring your HVAC system delivers the CFM it was designed to provide.
Why Flex Duct Has Higher Friction Loss
Flexible ductwork creates significantly more pressure drop than rigid ductwork because of its internal construction. The helical wire reinforcement inside flex duct and the textured polymer liner create a turbulent boundary layer that opposes airflow. Research shows flexible duct typically creates 25–40% more pressure loss than equivalent rigid ductwork. When properly stretched and installed correctly, flex duct approaches the performance of metal duct; however, most field installations experience compression that degrades performance significantly.
Compression and Sagging: The Hidden Capacity Killer
Installation quality has a dramatic impact on flex duct performance. Even modest compression—as little as 4%—can reduce CFM capacity substantially. Research indicates that at just 4% compression, flex duct delivers 37% less CFM than metal duct of the same diameter. At 15% compression, friction rates double, and at 30% compression, they quadruple. Sagging between support points has an even worse effect, potentially increasing friction loss by 200–300% in severe cases.
The physics is straightforward: when flex duct sags or compresses, the effective internal diameter constricts, forcing air through a smaller, irregular cross-section. This creates turbulence and choking that dramatically reduces airflow. To minimize this effect, flex duct should be stretched to its full length and supported every 4 feet maximum. Even with perfect installation, you typically need a flex duct one to two sizes larger than the equivalent metal size.
Impact of Bends and Elbows
Each bend or elbow in a flex duct run adds equivalent friction length and reduces available CFM. A 90-degree bend in flex ductwork adds approximately 25–30 equivalent feet of straight duct friction. The more bends you have in a run, the greater the total equivalent length, and the lower the final CFM. This is why the Duct Size Calculator includes bend factors in its sizing methodology. Minimizing the number of direction changes in your duct layout is one of the most effective ways to preserve CFM.
Standard Derating Factors
HVAC calculators apply an automatic derating factor of approximately 15% when flex duct is selected, reflecting the baseline loss in friction. However, this baseline assumes the duct is stretched and properly supported. If your installation includes compression or sagging, the actual derating is much higher. Some engineering references break flex duct performance into three compression states: 4% compression, 15% compression, and 30% compression, each with significantly different CFM values. When in doubt, use a larger duct size than the calculator suggests, or consult the sizing charts for your specific duct brand and compression state.
Practical Derating Guidance
In practice, minimize CFM loss by stretching flex duct fully, supporting it at 4-foot intervals, and keeping bends to a minimum. Avoid coiling excess duct or draping it loosely, as even small sags compound friction loss. If your layout requires multiple bends or long runs, consider upgrading to the next larger duct size beyond what a standard calculator suggests. For critical supply or return runs, rigid ductwork is worth the installation cost because it eliminates compression and sagging losses entirely.
When you input your duct dimensions and friction rate into a sizing tool, remember that flex ductwork will deliver less CFM in the real world than the calculator predicts, especially if installed with any slack, compression, or kinks. Use this guide as a reminder to account for installation reality, not just theoretical capacity.