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What is a Variable Air Volume System (VAV)?

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446 views asked Sep 10, 2014 by Mepanet

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VAV system consists of central air handling units with VFD's on the supply and return fans, ductwork and VAV boxes (Terminal Air Units). There are two VAV systems, single duct cooling only with reheat coil (electric or hot water) on terminal air units for each exterior zones or dual duct with hot duct for exterior zones and cold duct for both interior and exterior zones. In some dual duct systems, the hot duct will be used for cold duct in summer time as well instead of having a large cold duct.

Air Flow Requirements

In variable volume system, the load should be estimated properly, if the room net loads are not determined accurately, a variable air volume system will furnish too much air to the room, causing partial throttling at full load and excessive throttling at part load. Over sizing of fans, ducts and terminal air units should be avoided. 

Maximum Zone Air Flow Requirements

Zone duct work and VAV boxes (Terminal Air Units) must be sized according to the peak load which is governing factor. This can be either summer or winter sensible loads, latent loads, ore ventilation loads, whichever is the greatest combination.

Maximum Fan Air Flow Requirements

The airflow required at the sum of all peak loads from the governing latent or ventilation loads (which exceed the volume required in any zone to satisfy the summer sensible heat in that zone during the time) when the entire system reaches its peak sensible load for the total building.  As each zone peaks at a different time of day, it borrows the extra air from off peak zones, thus airflow of fan in central air handling unit should be sized per block load.

Total fan airflow (CFM) system considerations also should include:

  1. Leakage through outside air and relief air dampers.
  2. Building stack effect.
  3. Building exfiltration or infiltration
  4. Exhaust air system (when drawing from conditioned spaces)

The amount of 1-1.1 CFM/SF for block load is practical (which covers leakage, infiltration, etc), the amount of exhaust air from space will be provided by outside air intake in the air handling unit.


answered Sep 21, 2014 by Mepanet

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