Breaking Down a VAV HVAC System

Team Engineering was recently hired to assess the condition and functionality of a complex VAV system in a large commercial building. This inspired us to brush up on this special type of system.

Let’s start with the basics: VAV stands for Variable Air Volume. VAV systems typically have three primary components:

1. Roof top units (or other air handling unit), which provide heating (either gas or electric) and cooling,
2. VAV boxes, and
3. Ductwork

Each heating/cooling zone in the building will have a thermostat and associated VAV box. The VAV box features a damper which can modulate between different positions to allow different amount of airflow into the zone to control the temperature. Some VAV boxes may also contain a coil for heating (and in some cases cooling) where specific zones, such as those at the perimeter of the building, may have higher heating (or cooling) requirements. Depending on the size and number of VAV boxes, the system controls can become complex. It is important to have communication between the boxes and the roof top unit to achieve adequate supply air temperature as well as appropriate supply air volume.

For example, if all zones are calling for max airflow, the roof top unit fan should operate at max capacity to meet the demand. Fresh air is typically incorporated via economizer at a roof top unit or some location at the return ducting. This type of system has been historically very popular, especially for large commercial buildings. It excels at providing a fair degree of localized temperature control at a moderate cost. However, it struggles to serve a range of heating and cooling demands within the system as the supply air is typically all delivered at the same temperature. There are some variations of VAV systems that allow of simultaneous heating and cooling, but these systems quickly become expensive and complicated which defeats the advantages of the VAV system. The graphic below provides a simple visual of a VAV system.