Building Management: Variable Air Volume (VAV) and Control

Building Management: Variable Air Volume (VAV) and Control

Variable air volume (VAV) includes an assembly of valves and dampers in a single enclosure usually installed in a HVAC duct or overhead pipes.

VAV box/terminal is part of HVAC system for regulating airflow to a room/zone in response to a signaling device, usually a thermostat or sensors installed in a zone.

The difference between VAV and constant air volume (CAV) systems is that in VAV, temperature of the supplied air is constant while the volume is varied. In CAV, volume of the supplied air is kept constant. Most modern buildings use VAV systems for better energy savings. 

VAV systems can be:
  1. None fan less: Includes a controller and an actuator for damper control. A thermostat in the zone connected to the controller determines the volume of the air allowed by the damper opening.
  2. Fan powered; Same to the non-fan less but with a fan to pull used air from the room. This tries to maintain constant air in the room when damper opens and closes as well as reduces demand of heating air.
  3. Fan less with pre-heat: Includes a preheat coil to warm air. This is usually found with VAV systems in Europe and North America. 



Sample Carrier 35E Single-Duct VAV Terminal


VAV terminal components includes:
  1. Actuator
  2. Damper
  3. Controller
  4. Thermostat


An example of the VAV controllers is the BAC-8005. These can also be connected to a Building Management System for remote management via BACnet MS/TP.

VAV boxes usually have airflow sensor (for measuring pressure change across inlet duct to calculate flow rate) on the cross section of the VAV input duct. With the BAC-8005 controller, if the VAV box does not have flow pickups, you would need any of these sensors (SSS-1014 and SSS-1015) depending on the length requirements.

Sample VAV thermostats or sensors that can be connected to the VAV controllers for new installation or retrofitting purpose include:
  1. STE-8001W80: for use with SimplyVAV (KMC Controls) controllers
  2. RDG400: features DC 0-10V outputs and 3 position damper outputs.
  3. VT7200C5031: Features 1 or 2 floating outputs and 1 or 2 on/off outputs.

See below a sample wiring diagram for Viconic VT7200F500 thermostat used for controlling heating and cooling with automatic changeover for a VAV system. 


For automatic changeover in airflow temperature, a discharge air temperature (DAT) sensor is required. For VAV units with reheat, the DAT sensor is better installed after the reheat unit. An example is the KMC controls' STE-1401 DAT sensor.

Contact our HVAC Controls Engineering team for assistance with queries on residential and commercial HVAC systems and controls. 

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