VAV (Variable Air Volume)
VAV — variable air volume — conditions zones by modulating how much air each zone box admits, rather than reheating constant air; the central fan then slows to match total demand, cutting fan energy while every zone holds its own setpoint.
Why it matters on an MEPF project
Constant-volume systems ride one airflow all day and fight diversity with reheat or overcooling; VAV lets a west-facing meeting room and a shaded corridor each take what they need. Because fan power falls steeply as flow reduces, VAV's part-load economics is where it earns its complexity — offices with variable occupancy are its home ground.
It succeeds or fails on controls discipline: box calibration, duct static-pressure reset logic, and minimum-flow settings that keep ventilation honest at low load. A VAV system with untuned minimums quietly becomes a constant-volume system wearing dampers.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Zone device | VAV box with damper (+ optional reheat) per zone | system design |
| Fan control | VFD on supply fan tracking duct static setpoint | controls design |
| Minimum flow | Set to preserve fresh-air ventilation at low loads | ventilation basis (NBC Part 8) |
| Payback driver | Fan-energy reduction at part load + zone comfort | ECBC-era practice |
Common mistakes
- Zones drawn by geometry rather than load similarity.
- Static setpoint fixed high forever — the VFD never actually saves.
- Minimum flows set to zero, starving ventilation in mild weather.
- Commissioned once, never re-balanced after fit-out churn.
Related on this site
Frequently asked
Is VAV worth it for a small office?
Below a certain scale the controls overhead outweighs the fan savings — simpler zoning or VRF often fits better. Diverse mid-to-large floor plates are where VAV pays.
Why is one VAV zone always hot?
Usually an under-flowing box: mis-calibrated, damper stuck, or the static reset starving the index run. Trend the box data — VAV systems tell on themselves if anyone reads them.
Standards referenced: NBC 2016 Part 8 (ventilation basis) · ECBC 2017