Variable Air Volume (VAV)

What is VAV in HVAC?

Variable air volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system that regulates airflow to different zones in a building to meet specific heating or cooling demands. Unlike constant air volume (CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant or varying temperature.

How Does a VAV System Work?

A VAV system adjusts the amount of air delivered to a space based on its heating or cooling requirements. The key components include an air handling unit, VAV boxes or terminal units, and a variable frequency drive (VFD).

• Air Handling Unit (AHU): The AHU cools or heats air and supplies it through ducts to various zones. The air is commonly supplied at around 55 degrees Fahrenheit.
• VAV Boxes/Terminal Units: Each zone has a VAV box with a damper that modulates airflow. The damper position is adjusted to meet the temperature requirements of the zone. A thermostat in the zone signals the VAV terminal to adjust the airflow.
• Variable Frequency Drive (VFD): The fan in the central unit utilizes a VFD to adjust the amount of air delivered based on the cumulative system demand from the zones. As VAV boxes open or close, a static pressure sensor in the main supply duct detects pressure changes and signals the VFD to adjust the fan speed.

In cooling mode, when the preferred temperature in a space is reached, the VAV box closes to limit cool air. As the temperature increases, the box opens to bring the temperature back down. The supply air fan is regulated by a variable-speed drive, which controls the air volume by maintaining a constant duct static pressure.

Applications for Variable Air Volume

VAV systems are effective in medium to large-scale buildings with multiple HVAC zones. Common applications include the following:

• Office buildings
• Schools and universities
• Hospitals and healthcare facilities
• Apartment buildings
• Retail spaces
• Open-space HVAC applications such as restaurants and auditoriums

VAV systems are particularly well-suited for buildings where different zones experience significant variations in heating and cooling loads throughout the day.

Benefits of VAV Systems

• The ability to reduce fan energy at partial loads makes VAV systems energy efficient. 
• Precise temperature control in each zone ensures comfort for building occupants.
• VAV provides flexibility to adapt to changing occupancy and usage patterns.

Resources for VAV

• Trane Variable-Air-Volume Units
• Video: Trane Engineers Newsletter LIVE: Energy-Saving Strategies for Rooftop VAV Systems
• Video: Trane Engineers Newsletter Live: Single-Zone VAV Systems
• Customer story: Merrick & Company
• Customer story: Lakeview Hospital

VAV systems are an example of modern, efficient commercial HVAC design, offering performance and sustainability advantages in environments where comfort and energy use matter most. To discover if a VAV system will work for your facility, talk to your local Trane representative.