Duct velocity is the speed at which air moves through ductwork, measured in feet per minute (FPM). Velocity directly affects noise levels, friction losses, and system efficiency. Proper velocity selection balances airflow distribution with acceptable noise and energy consumption.
Recommended Velocities
Residential supply ducts should maintain 400 to 900 FPM, with main trunks at 700 to 900 FPM and branch ducts at 400 to 600 FPM. Return ducts should not exceed 600 to 800 FPM to reduce noise. Higher velocities increase friction rate and pressure drop exponentially, requiring larger motors and consuming more energy.
Practical Applications
Excessive velocity causes duct noise, whistling at registers, and wasted energy. Insufficient velocity results in poor air distribution and inadequate conditioning. Velocity is calculated by dividing airflow (CFM) by duct cross-sectional area. Proper duct sizing ensures optimal velocity throughout the system for comfort and efficiency.