Inches Water Column (IWC) is a unit of pressure measurement commonly used in HVAC systems to quantify static pressure, velocity pressure, and total pressure in ductwork. One inch of water column equals 249.09 Pascals. This unit is preferred in North American HVAC because it relates directly to water manometer measurements.
Measurement Standards
Standard residential HVAC systems operate at 0.1 to 1.0 IWC of static pressure. Commercial systems may reach 2.0 to 4.0 IWC depending on duct design and filtration. Velocity pressure in typical supply ducts ranges from 0.05 to 0.15 IWC, measured at 400-600 feet per minute airflow.
Testing and Balance
Technicians use digital manometers and pitot tubes to measure pressure in IWC units. Proper pressure measurements ensure balanced airflow throughout the system. Excessive pressure, above 1.5 IWC, indicates duct obstruction, filter loading, or sizing issues requiring corrective action.