VAV Is the Dominant HVAC System in Commercial Office Buildings
Variable Air Volume is the most widely used HVAC system type for commercial office, educational, and healthcare buildings. Understanding how a VAV system works - from the central air-handling unit through the ductwork to the terminal VAV boxes - helps architects make informed decisions about ceiling heights, shaft sizes, mechanical room locations, and equipment coordination. The ARE PPD tests VAV at a conceptual level: how the system works, what its advantages are, and what its spatial requirements are.
How a VAV System Works
A VAV system has two main components: the central Air-Handling Unit (AHU) and the zone-level VAV terminal boxes.
The Air-Handling Unit (AHU) conditions air centrally. It draws return air from the building and mixes it with outdoor fresh air (for ventilation), passes the mixed air through cooling coils (chilled water in a large building, DX refrigerant in a smaller system), then through supply fans that push the conditioned air into the supply duct system. The AHU maintains the supply air temperature at a constant setpoint (typically 55°F for cooling). Large buildings have multiple AHUs serving different zones or floors.
VAV Boxes are the terminal units at each zone. Each box has a damper that modulates the volume of 55°F supply air delivered to the zone based on the zone thermostat's call. When a zone is hot, the damper opens; when the zone is near setpoint, the damper partially closes. Most VAV boxes also have a heating coil (electric, hot water, or steam) that activates when the zone is in heating mode and the VAV box is at minimum airflow.
VAV System Advantages
- Energy efficiency: The supply fan varies its speed (via VFD - variable frequency drive) to match the reduced static pressure when many VAV boxes are at minimum position. This can save 50–70% of fan energy compared to constant-volume systems.
- Zone control: Each VAV box controls its own zone independently - one side of a building can be cooling while the other is heating.
- Ventilation control: The central AHU provides 100% filtration and humidity control for all the air delivered to occupied spaces.
- Flexibility: Reconfigurations require only moving VAV boxes and ductwork - the central system is unchanged.
Space Planning Implications
VAV systems require significant space planning coordination:
- Mechanical room: AHUs are large - a 50,000-sf floor plate might require an AHU the size of a small room. Mechanical room sizing must be coordinated with structural bays.
- Shaft space: Supply and return duct mains run vertically through shafts and horizontally in the ceiling plenum. Shafts typically occupy 2–4% of the building floor plate on each floor.
- Ceiling height: VAV ductwork in the ceiling plenum requires typically 18–24 inches of plenum depth (plus structure above and ceiling below), driving floor-to-floor heights in fully ducted VAV systems.
Reheat VAV vs. Standard VAV
Standard VAV boxes reduce airflow as cooling demand drops - which can create problems when airflow drops below the minimum needed for ventilation. Reheat VAV boxes maintain minimum airflow and add heat when the zone needs warming. Reheat VAV provides better humidity and ventilation control but uses more energy than standard VAV.
Key Exam Points
- VAV: central AHU conditions air at constant temperature; VAV boxes vary volume at each zone.
- VFD on supply fan reduces energy consumption when boxes are at partial load.
- VAV boxes need heating capability for perimeter zones in winter (reheat or baseboard).
- Shaft and plenum space planning is critical for VAV systems - typically largest ductwork of any system type.
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