FSP0003 – Variable Air Volume (VAV) – Facility Science Podcast #3

By | May 14, 2019

Notes for FSP0003 – Variable Air Volume (VAV)
VAV Stands for Variable Air Volume (multi zone)
  • System design that allows conditioning multiple spaces each with independent heating, cooling, and air flow requirements with a single primary supply of conditioned air (default to talking about cooled air).
  • Example: Hypothetical building has 4 cooling zones each with its own thermostat but only one primary air handler and cooling plant for the whole building:
    • Hallway with a few offices – relatively constant demand for cooling during business hours.
    • Employee break area. – high demand for cooling during common break times, but low during other hours.
    • Storage area – low demand most of the time.
    • Production area – high demand during each production shift.
  • Theory of operation/interaction of parts:
    • Air handler/cooling/heating system supplies air that is a constant temperature into the main supply duct.
    • VAV boxes independently open or close depending on the demand for conditioned air in their serviced space.
    • Air handler adjusts the flow of air to attempt to keep the pressure or air flow in the main supply duct constant.
      • When more VAV terminal units are more open, the supply fan will need to move a higher volume of air into the main supply duct to maintain air flow or pressure.
      • When the VAV boxes are mostly closed the fan will need to move a lower volume of air into the main supply duct to maintain air flow or pressure.
  • Parts of the system
    • Cooling/Heating System
      • Probably multi stage cooling to deal with differences in air volume
      • Heating can be done at main supply but is usually done at each zone.
      • Main supply heating/cooling probably doesn’t need to know about zones, just controls temperature of air in main supply duct.
    • Air handler (exclude cooling mechanism for simplicity)
      • Has fan to pull air from return duct and/or outside and pushes it into main supply duct.
      • Some mechanism to control the amount of air pushed into the main supply duct. Air demand is determined by measuring pressure or air flow in the supply duct.  Some Examples:
        • Recirculation or inlet dampers to restrict the amount of air allow
        • Variable pitch fan – impeller blade angle changes to reduce amount of air moved with a constant speed fan.
        • Various methods of fan speed control.  Common modern speed control method is VFD (Variable frequency Drive).
        • Others.
      • Doesn’t need to know anything about zones.  Only needs to know duct pressure (“static pressure”) or air flow.
    • Main supply duct
      • Carries air from air handler and branches toward each zone.
      • Each zone branch ends in terminal unit (VAV box) which separates the zone ductwork from the main supply duct.
      • Has temperature sensor and pressure or air flow sensors.
    • Terminal unit
      • One for each zone to separate the zone duct work from the main supply duct.
      • Has a motorized damper to control the amount of air allowed to pass through from the main supply duct to the zone duct.
      • Temperature sensor in the space allows for control of the damper.  In cooling scenario, as the space gets warmer the damper opens more and more to allow more cool air into the space.  As the space cools back down, the damper gradually closes to restrict air flow.  Continuous variable flow of air allows for a more constant space temperature than a constant volume system where the air supply is either on or off.
    • Zone ductwork branches throughout zone space and ends in supply grills to actually dump air into space.
    • Return air back to air handler/exhaust to outside.
  • Reheat (sometimes but rarely Recool):
    • Often a zone will need a minimum flow of air through the VAV box to make up for ventilation air that is exhausted out of the building.
    • When the the air in the space has reached the desired temperature but cold air is still flowing into the space, the space will get too cold.
    • To prevent overcooling, some VAV terminal units have a reheat (hot water, electric, whatever) coil to allow the excess cold air to be reheated.
  • Methods to supply Heating with VAV
    • Central Heat where primary air supply can be changed from cool to heat. Often this is just done for “morning warm-up” of a building that has been unoccupied during the night but can be done on demand if designed that way.
    • Terminal heat.  Primary supply air is always cool.  Heating is done by:
      • Remote heating equipment in the space (radiators, etc).
      • Reheat coils in the VAV boxes.
      • VAV boxes that mix warm plenum (return) air into the cold supply air using fans and dampers in the VAV unit. Saves energy on reheat and can be used to maintain constant air flow into the zone.
    • Dual duct systems have 2 primary air supplies, one hot and one cold.  The VAV terminal units mix air from the 2 sources to achieve the desired temperature in the space.  Can be more expensive to operate but allow for precise temperature control.
  • Issues to look for (not a comprehensive list):
    • Clogged reheat coils:  Some VAV boxes have no provision to access the interior and clean the coil.  Check your boxes for proper air flow at the given valve position.
    • Excessive reheat (running the boiler in the summer) can silently waste a lot of energy (everybody will be comfortable and no alarms will sound).
      • Reheat should only be active when the air damper is at the minimum position.  A wide open box with active reheat indicates a malfunction.
      • Mis-sized boxes and/or inappropriate minimum air flows can cause an excess of reheat.
      • Un-optimized control system parameters (supply air temperature setpoint, heating deadband, static pressure setpoint, etc) can allow for excessive reheat.
    • Change in space occupancy can unbalance a VAV system.  During design, VAV boxes sizes are chosen based on the expected heat load.  If, for example, a storage room is turned into a machine shop the existing terminal unit will not be able to provide enough cold air to the space which may cause overcooling (and consequently excessive reheating) of the rest of the building as the system tries to satisfy the “broken” zone. Can be fixed by modifying the system to reflect the new occupancy (replace with larger terminal unit, etc, ask your engineer).
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