When to use: Use to size supply and return air ducts using the equal friction method per ACCA Manual D. Enter the required airflow (CFM) for the duct run and select the friction rate and duct material. The recommended friction rate is 0.08–0.10 in.wg/100 ft for residential systems. Maintain trunk duct velocities of 600–900 FPM and branch duct velocities of 400–700 FPM to minimize noise and pressure drop.