Pressure drop is the reduction in air pressure as it travels through ductwork, filters, and equipment components. Caused by friction and turbulence, pressure drop increases with velocity, duct length, and component restrictions. Excessive pressure drop reduces airflow, comfort, and system efficiency while increasing energy consumption.
Causes and Measurement
Friction against duct surfaces, sharp bends, transitions, and dampers all contribute to pressure drop. Measured in inches of water column (in. w.c.), acceptable pressure drop for residential systems typically ranges from 0.5 to 1.0 in. w.c. across the entire ductwork. Dirty filters can add 0.2 to 0.5 in. w.c. of additional pressure drop.
System Impact
Total system pressure drop determines blower motor requirements and operating efficiency. Undersized ducts or restricted filters force the blower to work harder, increasing electricity costs. Proper duct design and regular maintenance minimize pressure drop, improving comfort and reducing operating expenses.