Flex Duct Performance Calculator
Estimate air velocity and friction loss (static pressure drop) for flexible ductwork. Proper duct sizing is crucial for efficient HVAC system performance, comfort, and energy savings.
Enter your duct specifications below and click “Calculate Duct Performance”
Understanding Flex Duct Calculations
Flexible ducts are common in residential and light commercial HVAC systems due to their ease of installation. However, they typically have higher resistance to airflow (friction loss) compared to smooth metal ducts. This calculator helps estimate two key performance metrics:
- Air Velocity (FPM): Feet Per Minute. This is the speed of the air moving through the duct. Appropriate velocity is important to ensure proper air delivery without excessive noise or insufficient throw. Too high can be noisy; too low might not deliver air effectively.
- Friction Loss (in. WC): Inches of Water Column. This measures the static pressure drop as air moves through the duct due to friction against the duct’s inner surface. Higher friction loss means your fan/blower has to work harder, potentially reducing airflow to conditioned spaces and increasing energy use.
The calculator uses standard formulas to determine these values based on your inputs:
- Duct Cross-Sectional Area (A): Calculated from the diameter. `A = π * (Diameter / 24)^2` (square feet)
- Velocity (V): Calculated by dividing airflow by area. `V = CFM / A` (feet per minute)
- Friction Loss Rate (FR): Estimated for flexible duct per 100 feet using an empirical formula. A common approximation for flex duct (which is rougher than smooth metal duct) is: `FR ≈ 2.0 * 0.109136 * (CFM^1.9) / (Diameter_inches^5.02)` (inches WC per 100 ft). The factor of 2.0 (or similar) accounts for the higher roughness of typical flex duct compared to a baseline smooth metal duct.
- Total Friction Loss (TFL): Calculated by multiplying the friction rate by the duct length. `TFL = (FR / 100) * Length_ft` (inches WC)
Minimizing sharp bends and ensuring the duct is pulled taut (not compressed like an accordion) during installation is crucial for achieving performance closer to these calculated estimates.