Air Volume Flow (VavSuAirFl) and Relative Air Flow (VavSuAirFlRel) are calculated values computed from logic that utilizes the following:

  • Velocity pressure from the differential pressure sensor or other flow sensing technology
  • Units conversion constant to convert to air velocity
  • Flow coefficient value for calibration at a specific airflow setpoint
  • Duct area in units compatible with the units conversion constant
  • Nominal airflow value (user entered)
  • Hood air volume flow value to aid in calculating the correct flow coefficient value


First, air velocity is a calculated value computed from logic using the following formula:



  • AirVEngrgUnits is the velocity of the air in engineering units that are compatible with the other units used in the airflow formulas above
  • KEngrgUnits = constant for engineering unit correction [the “4005” (English units) or “1.29” (SI units)
  • CSpanCal = constant to calibrate for sensor span (also called flow coefficient)
  • Pdifferential = differential pressure sensor reading from duct


Next, Air Volume Flow in flow units (VavSuAirFl) is calculated by the formula:



  • Area = area of duct
  • FlCnv = conversion constant for converting between box dimensional engineering units and desired flow engineering units


Air Volume Flow as a percentage of the nominal airflow (Relative Air Flow [VavSuAirFlRel]) is calculated by the formula:



  • AirFlNom is the nominal air volume flow of the VAV terminal box.