VAV Terminal Box Product

Overview

Variable air volume (VAV) terminal boxes are demand-responsive air distribution devices that automatically adjust the volume of supply air to each room or zone based on thermal load and occupancy. At the core of a VAV box is a proportional damper that throttles airflow from the central air handling unit; as a room's cooling requirement decreases (due to lower occupancy, time of day, or a passing sunny period), the damper modulates to a smaller opening, reducing supply CFM and the load on the chiller. When heating is needed (below the space setpoint), a reheat coil energizes, warming the minimum airflow to achieve comfort. VAV systems enable significant energy savings compared to constant air volume (CAV) systems because the central plant (chiller and fans) operates at part load and can stage off multiple units, whereas CAV systems must continuously distribute full design airflow regardless of actual demand.

Terminal boxes range from simple ductwork-mounted boxes serving a single room or zone to more sophisticated units with integrated filters, acoustic dampening, and advanced control electronics. Modern boxes integrate BACnet or Modbus communication, allowing them to receive setpoints and send status signals to a building management system, enabling coordinated load balancing and peak demand response.

How it works

Demand sensing and damper modulation: A local temperature sensor in the room or return air duct measures space temperature. The VAV controller compares this to the setpoint (e.g., 72 °F) and outputs a 0–10 V signal to the damper actuator. If room temperature rises above setpoint (cooling demand exists), the damper opens fully (or to a setpoint maximum of 80–100% CFM). As the room cools toward the setpoint, the damper proportionally closes, reducing supply CFM. If the room temperature falls below setpoint (heating demand), the damper remains at its minimum position (typically 15–30% of design CFM), and the reheat coil is energized to warm this minimum airflow to the space setpoint.

Flow measurement: A differential pressure sensor with an averaging probe taps the velocity pressure in the supply duct and converts it to a calibrated CFM value. This measured flow is returned to the controller for verification and is transmitted to the building automation system. If measured flow deviates from the commanded value (e.g., due to dirty filters or damper stiction), the controller can alert maintenance.

Reheat control: Electric resistance coils are energized by a 120 V or 240 V contactor when the space temperature falls below setpoint and the damper is fully closed (minimum airflow position). Hot water reheat coils use a proportional two-way valve that modulates hot water flow as needed to reach the supply air setpoint (typically 85–95 °F). The reheat is staged to avoid energy waste: if the chiller can satisfy demand without reheat, the valve remains shut.

Minimum airflow enforcement: The controller enforces a minimum airflow setpoint (typically 15–30% of design CFM) to ensure outdoor air is delivered to the space for ventilation compliance. This minimum is set by the building code (ASHRAE 62.1) and is independent of thermal load; even if the room is perfectly cool, the VAV box will not close below the minimum to ensure fresh air.

Acoustic and isolation: The insulated casing and internal damper design provide sound attenuation, reducing the noise of high-velocity supply air. This allows the central air handler to operate efficiently without generating objectionable noise in occupied spaces.

Subsystems and integration

Damper and Actuator Assembly is the primary control element; its response time and accuracy directly affect occupant comfort and energy performance. Flow Measurement Sensor provides verification and feedback; drift or fouling is detected by monitoring flow vs. damper position curves. Reheat Coil Assembly converts excess ventilation air into warmth; careful valve tuning prevents short-cycling and reheat-cooling overlap where cold supply air is immediately reheated, a major energy waste.

Common failures

Damper blade stiction (sticky movement due to rust or sealant buildup) causes sluggish response and occupant complaints of temperature swings. Cleaning or lubrication usually restores function. Flow sensor blockage (dust accumulation on the probe opening) causes false low-flow readings and triggers unnecessary alarms. Cleaning the probe restores accuracy. Reheat valve stiction prevents proportional control; valve replacement or internal part cleaning is required. Control board failure (capacitor failure, relay pitting) stops modulation; board replacement is needed.

Commissioning and tuning

Upon installation, each VAV box must be balanced: the damper is set to its maximum position, and the supply ductwork is balanced to deliver design CFM. The flow sensor is then calibrated against a known flow standard (pitot tube traverse). The local controller's setpoint is compared to the room thermostat; if discrepancies exist, the sensor offset is trimmed. The damper response time is verified, and the minimum airflow setpoint is confirmed to match code requirements. Building system integration testing verifies that BACnet or Modbus status and commands are exchanged correctly.

Energy benefits

In a large office building, VAV systems typically consume 30–50% less fan energy than equivalent CAV systems because the central supply fan operates at part load and scales its speed to match total demand. Part-load chiller efficiency also improves. However, energy savings depend on proper commissioning: poorly tuned reheat valves and setpoints can negate all benefits. A well-commissioned VAV system is a hallmark of high-performance buildings.