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Mechanical / HVACaka: VAVaka: VAV systemaka: VAV box

Variable Air Volume

In Plain English

An HVAC system that saves energy by delivering more or less air to each zone depending on how much heating or cooling is needed.

Definition

An HVAC distribution system that varies the volume of conditioned air delivered to each zone based on the zone's current heating or cooling load, while maintaining a constant supply air temperature. VAV boxes modulate their damper position in response to the zone thermostat, reducing airflow when the load decreases. VAV systems are more energy-efficient than constant-volume systems because they reduce fan energy at part load.

Why It Matters in Bidding

VAV systems significantly affect mechanical bid value because each zone box, its controls, reheat coil, and ductwork branch must be counted and priced individually, making the takeoff far more granular than a constant-volume system. Estimators must coordinate the VAV box schedule with the controls and electrical scopes, since miscounting boxes or omitting reheat and DDC controls creates major cost gaps that surface during submittals or commissioning.

Example

On an office fit-out, the estimator counts 38 VAV boxes from the mechanical schedule, prices each with its hot-water reheat coil and DDC controller, and confirms with the controls sub which party carries the thermostats and wiring.

Related Terms

DamperAir HandlerBuilding Automation SystemVariable Frequency Drive

Frequently Asked Questions

Estimators count boxes from the equipment schedule and floor plans, matching each tag to its size, reheat type, and accessories. Each box is priced with its coil, actuator, controller, and connection labor. The branch ductwork, flex, and diffusers downstream are taken off separately so the box count and distribution costs do not overlap.
The biggest gaps are controls and electrical. VAV boxes need DDC controllers, thermostats, and wiring that may sit in the controls or electrical scope rather than mechanical. Reheat coils may need hot-water piping from the plumbing or mechanical piping crew. Estimators must clarify these splits in the bid to avoid double-counting or omissions.
VAV adds a controlled box at every zone, each with dampers, actuators, controllers, and often reheat, plus more sophisticated DDC sequencing and commissioning. That raises first cost and labor versus a single constant-volume unit. The tradeoff is part-load fan energy savings, which estimators may need to present in a life-cycle comparison during value engineering.

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