As we previously discussed in one of our building technology basics videos, VAV (Variable Air Volume) controllers are integral to HVAC systems that require varying cooling and heating loads in different zones. In this article, we will explore some of the most common types of VAV boxes and their specific applications. Of course, there are many one-offs and iterations of these applications, but these are the ones you’ll see most frequently.
Standard VAV Boxes
Standard, cooling-only VAV boxes consist of a VAV controller with an actuator that controls a damper. The VAV controller is also usually wired to sensors that measure pressure, temperature, and humidity at the inlet of the box and to a wall sensor in the zone that is being heated or cooled.
VAV with Reheat
VAV boxes are commonly equipped with reheat coils that heat the air going through the box when conditions call for it. These reheat coils can be either electric or hydronic. Regardless, they serve the same purpose of heating the air in the box before it is pushed into the space.
Fan-Powered VAV Boxes
Fan-Powered Boxes (FPB) consist of the same components as standard VAV boxes and also include a small fan. Where this fan is placed in the box determines whether it is a parallel FPB or a series FPB.
Parallel Fan-Powered Boxes
The term “parallel” for these units comes from the fan being placed outside of the primary airflow so that it is blowing in a parallel direction with the air coming in through the inlet. The fan in a parallel FPB pulls air from the plenum above the ceiling which is warmer than the air coming from the central unit (usually an air handler). Because of this, the fan typically runs during heating or deadband modes only.
Series Fan-Powered Boxes
The term “series” for these units comes from the fan being placed in series (or inline) with the primary airflow. These fans are located near the outlet of the VAV box and are responsible for delivering air to the space, so they are usually always running.
With parallel FPBs in cooling mode, in particular, the central unit is required to provide enough airflow to reach the space whereas, with series FPBs, the central unit can be downsized because the terminal unit will be delivering the airflow to the space. To learn more about pressure control in VAV systems and the air handler’s role in efficient VAV operations, check out the articles below:
Static Pressure Control and VAV Operation