Why VAV — and why now
Variable Air Volume distribution is the dominant commercial HVAC strategy because it saves substantial fan energy at part-load. A typical commercial office runs at 30-60% of peak cooling load most operating hours. Constant Air Volume systems run the fan at 100% airflow regardless; VAV reduces airflow proportionally to load. Fan energy scales with the cube of airflow — so a 50% reduction in airflow saves roughly 87% of fan power at that operating point by the fan laws (less in practice, because duct static pressure control and motor and drive losses remain). Most green building rating systems (LEED, Green Star, BREEAM, DGNB) reward the resulting energy savings.
VAV terminal types
Single-duct VAV (most common)
One supply duct delivers cold air at constant temperature (13-14°C). The VAV terminal box has a damper that modulates airflow to match zone cooling load. Returns to the AHU via the ceiling plenum or dedicated return duct. Most efficient and most common. Sub-types:
- Cooling-only: simple damper, no heating capability. Used for interior zones where heating is not required.
- Reheat (HW or electric): damper plus heating coil downstream. Reduces cooling airflow then activates reheat for low-load conditions or perimeter zones in winter.
- Pressure-independent: includes velocity sensor and feedback control to maintain commanded airflow regardless of duct static pressure variation. Standard for commercial.
Fan-powered VAV (FPB)
VAV box with built-in fan that mixes plenum return air with primary supply air. Used to maintain minimum airflow at low-load conditions while extracting heat from lighting and equipment in the plenum. Sub-types:
- Series fan-powered: fan runs continuously at all loads, primary damper modulates. Higher continuous fan energy.
- Parallel fan-powered: fan runs only at low primary airflow conditions. More efficient but more complex control.
Dual-duct VAV
Two supply ducts: one cold (13-14°C) and one hot (35-40°C). VAV terminal mixes the two streams to deliver any temperature between. High control flexibility but high fan energy and twice the duct mass. Used in some legacy buildings; rarely specified for new construction.
Induction VAV
Primary supply discharges at high velocity through a nozzle, inducing secondary plenum air through the terminal. Self-powered (no fan). Limited capacity; used in some hospitality applications and legacy retrofits.
VAV box sizing
Three flow rates per zone
- Maximum cooling airflow: sized to peak sensible cooling load with the design supply temperature (typically 13-14°C). Peak load includes solar gain, internal lighting/equipment/people loads, and envelope conduction at design conditions.
- Minimum airflow: set by ventilation needs (ASHRAE 62.1 or local code minimum) and the energy code's reheat limits; check diffuser mixing at this setting.
- Reheat-mode airflow: the airflow while the reheat coil is active. For single-duct reheat it is capped by the energy code's reheat limits; fan-powered boxes can add plenum air through the box fan without extra primary air.
Inlet diameter selection
VAV boxes have round duct inlets in standard diameters. Approximate flow range per inlet at typical pressure drop budgets:
- Φ150 mm: 50-200 L/s
- Φ200 mm: 100-400 L/s
- Φ250 mm: 200-700 L/s
- Φ300 mm: 350-1,100 L/s
- Φ400 mm: 600-2,000 L/s
- Φ500 mm: 1,000-3,500 L/s
Major manufacturers (Trox, Titus, Krueger, Price, Carnes, Tuttle & Bailey, Halton) publish detailed selection software that accounts for pressure drop, NC rating and damper authority.
Diversity factor
System-level fan and AHU sizing uses the system peak airflow which is less than the sum of all zone peak airflows. Reason: zones peak at different times. Eastern perimeter peaks at 9-11am (morning solar). Western perimeter peaks at 2-4pm. Interior peaks during peak occupancy. The fan never has to deliver every zone's peak simultaneously.
Typical diversity factors:
- Commercial office: 0.70-0.85
- Mixed-use commercial: 0.75-0.85
- Hospital (acute care): 0.85-0.95 (less diversity, more constant load)
- Hotel: 0.65-0.75 (high diversity, occupancy varies widely)
- Retail: 0.70-0.80
- Educational: 0.60-0.75 (occupancy seasonal, classrooms peak at different times)
Detailed building energy modelling (eQUEST, EnergyPlus, IES VE, TAS) calculates project-specific diversity. Industry rules of thumb apply when energy model is not available.
Minimum airflow per ASHRAE 62.1
ASHRAE 62.1 specifies minimum outdoor air ventilation per zone based on floor area and occupancy. Typical office values:
- Office space: 0.06 cfm/sqft floor + 5 cfm/person (≈ 0.30 L/s/m² + 2.5 L/s/person)
- Conference room: 0.06 cfm/sqft + 5 cfm/person (higher occupancy density drives this)
- Classroom: 0.12 cfm/sqft + 10 cfm/person
- Hospital patient room: 0.18 cfm/sqft + 25 cfm/person plus 6 ACH minimum
- Retail: 0.12 cfm/sqft + 7.5 cfm/person
VAV minimum setting must equal or exceed the calculated ventilation requirement at all times. Multi-zone systems with shared AHU use ASHRAE 62.1 multi-zone calculation that adjusts outdoor air at the AHU based on zone-level occupancy diversity.
Pressure class for VAV duct
Supply duct upstream of VAV boxes operates at higher static pressure than downstream:
- Upstream of VAV (between fan and box inlet): SMACNA +3 to +4 in.wg / EN 1507 air tightness Class C / AS/NZS Class C. The fan delivers static pressure required to feed every VAV box plus duct losses.
- Downstream of VAV (between box outlet and diffuser): SMACNA +1 in.wg / EN 1507 Class A / AS/NZS Class A. The VAV box drops the pressure to diffuser-feeding levels.
Fabrication implication: supply mains need heavier gauge (typically 20-18 gauge for medium pressure) and tighter sealing than branch ducts (22-24 gauge for low pressure). The Taokron SBAL-V auto duct line stores PLC recipes for both pressure classes; the operator selects per duct section. This avoids over-fabricating low-pressure branches as medium-pressure construction.
Acoustic considerations
VAV systems generate noise from three sources:
- Fan and AHU: dominant low-frequency source, controlled with attenuators near AHU and lined plenum chamber
- VAV box itself: damper modulation creates regenerated noise, increasing as damper closes from full-open. Typical 25-40 dB(A) at maximum airflow, increasing as box throttles down.
- Air diffuser: terminal device noise, controlled by diffuser selection and face velocity
VAV box noise typically dominates at the zone level. Selection must check NC rating at all operating points, not just at peak airflow. Many projects use acoustic-lined VAV inlet ducts (300-600 mm of internally lined duct upstream of box) to attenuate fan noise propagating through the box. See our acoustic duct lining guide.
Common VAV design errors
- VAV minimum set without checking diffusers: very low minimums cause poor diffuser mixing and unstable damper modulation. Set the minimum from ventilation needs (ASHRAE 62.1) and the energy code's reheat limits, then confirm diffuser performance at that airflow.
- Reheat airflow above the energy code limit: ASHRAE 90.1 limits how much air may be cooled and then reheated. Watch the project specification for compliance audit.
- Insufficient inlet duct upstream of VAV: less than 3 duct diameters of straight inlet causes flow distortion and pressure-independent control failure. Always provide 3D minimum, 5D ideal.
- Wrong pressure class on downstream duct: fabricating low-pressure box-to-diffuser duct as medium-pressure wastes 30-40% on fabrication cost. Conversely, low-pressure construction upstream of VAV fails leakage class.
- Diversity factor not applied at system level: oversized fan and AHU. Wastes capex and runs at low efficiency at design point.
- Diversity applied at branch level: undersized branch ducts. Each branch must be sized for that zone's peak, not the system peak.
Get an SBAL-V quote configured for VAV pressure classes →
FAQ
What is a VAV system?
Variable Air Volume — supply temperature constant (13-14°C), zone airflow modulated by VAV terminal box to match cooling load. Fan energy falls steeply at part load (about 87% less fan power at half flow by the fan laws, less in practice).
What is diversity factor?
Adjustment for non-coincident zone peaks. System fan/AHU sized to system peak (70-85% of sum of zone peaks for office). Branch ducts sized for zone peak (no diversity).
How do I size a VAV terminal box?
VAV box sizing requires three flow rates per zone: maximum cooling airflow (sized to peak sensible cooling load with 13-14°C supply), minimum airflow (set by ventilation needs and the energy code's reheat limits), and reheat-mode airflow (the airflow while the reheat coil is active, also capped by the energy code's reheat limits). VAV box catalogue selection then matches inlet diameter to required airflow at the desired pressure drop budget (typically 50-150 Pa). Round inlet diameters: Φ200 mm for ~400 L/s, Φ250 mm for ~700 L/s, Φ300 mm for ~1,100 L/s. Major VAV box manufacturers: Trox, Titus, Krueger, Price, Carnes, Tuttle & Bailey.
What is the minimum VAV airflow setting?
Minimum airflow is set by ventilation needs and the energy code's reheat limits. The ventilation floor is the ASHRAE 62.1 requirement for the zone (for office space 0.06 cfm/sqft + 5 cfm/person, about 0.3 L/s per m² + 2.5 L/s per person) or any stricter local code (e.g. AS 1668.2 in Australia). The energy code (e.g. ASHRAE 90.1) limits how much air may be cooled and then reheated, so the minimum should not be set higher than ventilation and diffuser performance need. Check diffuser mixing and throw at the minimum setting, since very low airflow can cause poor mixing and comfort complaints.
What pressure class does VAV duct require?
Supply duct upstream of VAV boxes is typically Medium Pressure (SMACNA +3 to +4 in.wg / EN 1507 air tightness Class C / AS-NZS Class C) because the VAV box requires 50-200 Pa minimum static pressure to operate properly across its flow range. Downstream of the VAV box (the box-to-diffuser run), Low Pressure (SMACNA +1 in.wg / EN 1507 Class A / AS-NZS Class A) is sufficient. Fabrication implication: supply mains and risers need higher gauge and tighter sealing than branch ducts. Taokron SBAL-V recipes include both pressure classes; operator selects per duct section.
