Duct Design Principles
Proper ductwork design is essential for HVAC system performance and energy efficiency. The duct system must deliver the required airflow to each room while minimizing pressure losses and noise. The equal friction method sizes ducts to maintain constant pressure loss per foot of duct length, typically 0.08 to 0.12 inches of water column per 100 feet. The static regain method uses larger ducts near the fan to reduce velocity and recover static pressure as the air moves through the system.
Duct sizing calculations begin with the room-by-room load calculation that determines the heating and cooling requirements for each space. The required airflow in cubic feet per minute for each room is calculated based on the heating or cooling load. The duct size is then determined from the airflow and the recommended velocity. Main trunk ducts typically operate at 700 to 900 feet per minute, while branch ducts operate at 400 to 600 fpm.