Air Balance VAV System

Procedure to air balance vav system.

Variable volume systems are used in office buildings, shopping malls and in general buildings where the cooling load shifts rapidly. These systems change the air volume to fit the cooling requirements instead of varying the air temperature. Variable air systems can be medium or low pressure.

Check and note the following items on the supply and return fans.

  • Correct fan rotation.
  • Filter condition.
  • Cooling coil condition.

    Set the controls for the supply and return fans to operate at maximum capacity and for the diversity factor proportionate number of VAV boxes to be at the full open position. This must be done before starting.

    Set the system to operate with maximum return air and minimum outside air to air balance vav.

    The following preliminary data should be obtained at the supply and return fans.

  • Fan and motor RPM.
  • Motor current and voltage.
  • Fan, coil and filter static pressure,
  • Nameplate data on the fans and motors.
  • Motor sheave, fan pulley and belt sizes.

    Noting the systems design diversity factor set that number of boxes to a call for full cooling. A diversity factor takes into account that it is improbable that all VAV boxes will be full open at one time.

    In this manner the supply and return fans can be of a smaller capacity and more energy efficient.

    Air Balance VAV Procedure

    Traverse the main supply duct and return duct to determine cfm delivery of the fans to air balance vav system,

    Adjust fan speed control or change sheaves to ensure the supply and return fans can achieve the required full air flow.

    The return fan controls should be set that the fan returns 85% of supply air. The other 15% is made up of outside air known as minimum outside air. If this "tracking" is correct the mixed air chamber will just be ever so slightly negative when you try to open the access door.

    Adjust the supply and return fan to approximately 5% above design CFM.

    Go to the end of the longest run and check the air volume through the box and check against design parameters.

    Lower the supply fan discharge air pressure to the point where the desired CFM can be met with the VAV box at 90% open. This should then be the VAV system operating pressure.

    Start balancing the VAV boxes closest to the discharge of the supply fan.

    Set the box to a call for full cooling. Measure actual air volume and compare to the BAS system indicated air volume. If not the same adjust the VAV box flow coefficient until actual and indicated air volume is the same.

    Then air balance each supply air diffuse to it's design value.

    Set the each box to a call for heating or minimum cooling and measure and record diffusers minimum air flow.

    Air balance all above ceiling return air ducts to the correct return air volume using the duct balancing dampers. Start this balance at the return air opening closest to the return fan and work away.


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