Friction rate is the pressure loss per 100 feet of ductwork due to air resistance against duct surfaces. Measured in inches of water column (in. w.c.) per 100 feet, friction rate depends on duct diameter, air velocity, and roughness. Proper friction rate calculation ensures efficient HVAC system design and prevents excessive energy consumption.
Technical Specifications
Friction rate typically ranges from 0.02 to 0.10 in. w.c. per 100 feet for residential systems. Standard practice targets 0.04 to 0.06 in. w.c. per 100 feet to balance duct size and system efficiency. Higher velocities increase friction rate exponentially, following the friction loss equation used in ASHRAE guidelines.
Design Applications
HVAC designers use friction rate charts and software to size ducts appropriately. Lower friction rates require larger diameter ducts, reducing pressure drop but increasing installation costs. Higher friction rates use smaller ducts but demand more blower power and energy consumption. Balancing these factors determines optimal system performance and lifecycle costs.