Definition of VAV Boxes

Purpose of a VAV Box

• A VAV box can be installed in each group of rooms that experiences the same heating or cooling loads, which are also called thermal zones. They are designed to vary the airflow for the specific zone in which they are installed, in order to meet the VAV box's associated thermostat's temperature setpoint.

VAV Box Components

• A VAV box is made up of the following components: the casing, which houses the VAV box components; the inlet, which is a round connection from the supply ductwork to the VAV box; the damper, which can increase or decrease the volume of air that passes through the VAV box; the damper shaft, which allows the damper to open or close and is connected to a motor; the fan, which is a component of a fan powered VAV box and improves air velocity; the reheat coil, which can be added to a VAV box to improve air velocity and volume as a room nears its design temperature; and a thermostat, which allows the user to control the temperature.

Energy Savings with VAV Box

• When airflow varies between multiple zones, the unit fan that manages the main air flow can be turned down to conserve energy. Since airflow varies with changes in solar heat temperatures and ambient heat, the greatest energy saving in conjunction with terminal units generally occurs at the perimeter zones. The VAV box can also modulate airflow to conserve energy at partial load in other areas, when cooling or heating requirements change. VAV boxes can also include electric or hot water reheat coils if particular zones in a building require simultaneous cooling and heating, to provide heat simultaneously with the primary air or recirculated room air.

Pressure Dependant and Independant VAV Boxes

• VAV boxes can operate as either pressure dependent devices, in which the terminal unit uses a space temperature sensor to control the damper position; or pressure independent, in which the volume of primary air to the space is controlled via feedback from an airflow-measuring device in the VAV box. Pressure independent VAV boxes are most common because they enable the space temperature sensor to solicit airflow quantity in connection with the airflow-measuring device. This enables more stable control of airflow.