Duct friction refers to the resistance air encounters as it flows through ductwork, causing pressure loss and reduced system performance. This resistance increases with duct length, decreases with duct diameter, and varies based on surface roughness and air velocity. Understanding duct friction is essential for proper system design and capacity verification.
Friction Loss Factors
Friction loss per 100 feet of equivalent length ranges from 0.02 inches of water column at 400 CFM in 12-inch round ducts to 0.50 inches for the same airflow in 6-inch ducts. Aspect ratio significantly affects friction: flat high-ratio ducts experience 15-25% greater friction losses compared to round ducts of equivalent area. Flex ductwork exhibits 20-40% higher friction than rigid sheet metal due to inner surface roughness.
System Impact
Excessive duct friction reduces air delivery to remote registers and forces the fan to consume additional energy to overcome resistance. HVAC designers use friction loss charts and software to maintain acceptable pressure drops, typically 0.10 inches of water column per 100 feet of equivalent length. Undersized ducts create friction problems that cost 10-20% annually in wasted energy and inadequate comfort. Proper duct sizing minimizes friction while controlling installation costs.