ℹ About This Calculator
A Variable Air Volume (VAV) box throttles supply air to match each zone's changing cooling load. This calculator sizes the box from the zone cooling load and supply-air temperature difference, giving the design airflow, the turndown minimum and the inlet size, following ASHRAE and SMACNA practice.
Design airflow comes from the peak cooling load; the minimum airflow (typically 20-40% of design) must still meet ventilation and prevent dumping. Selecting the box on inlet velocity keeps noise low - the inlet is usually chosen so the box runs mid-range on its capacity, not at the top of its airflow band where it is noisy.
VAV Box Sizing Formula
ASHRAE / SMACNA
Q = load / (1.21 × ΔT) (SI, L/s) Q_min = Q × turndown Q = design airflow, L/s load = zone sensible cooling load, W 1.21 = air density × specific heat factor ΔT = supply-to-room temperature difference, °C turndown = minimum airflow fraction (0.2-0.4)
Worked Example
Example: A zone with a 3.5 kW (3500 W) sensible cooling load and an 11 °C supply-to-room ΔT needs Q = 3500 / (1.21 × 11) ≈ 263 L/s (about 560 CFM). At a 30% turndown the minimum is about 79 L/s. This suits a 250 mm (10 in) inlet box running comfortably below its noisy top-end airflow.
VAV Box Inlet Sizes & Airflow Ranges (ASHRAE / SMACNA)
VAV boxes are sold by inlet size; each covers an airflow band. Select so the design airflow sits in the mid-range, not the top, to limit noise:
| Inlet size | Typical airflow (L/s) | Typical airflow (CFM) |
|---|---|---|
| 150 mm (6") | 25 – 140 | 50 – 300 |
| 200 mm (8") | 50 – 250 | 100 – 530 |
| 250 mm (10") | 85 – 400 | 180 – 850 |
| 300 mm (12") | 120 – 570 | 250 – 1200 |
| 350 mm (14") | 165 – 800 | 350 – 1700 |
Minimum Airflow
The minimum setting must still deliver the zone's required outdoor air and avoid cold-air dumping at the diffusers. 20-40% of design is common; pressure-independent boxes hold the setpoint as duct pressure varies. Oversizing the box forces a very low turndown that can be noisy and unstable.
Indicative bands - confirm against the selected manufacturer's catalogue and NC data.
Frequently Asked Questions
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