A Pressure Dependent Variable Air Volume (VAV) system is a type of VAV configuration in which the airflow delivered by a terminal unit fluctuates directly with changes in duct static pressure, even when the damper position remains constant. The VAV box controller modulates the damper based on a temperature signal from the zone thermostat, but it has no feedback mechanism to verify or correct actual airflow. As a result, the volume of conditioned air reaching the zone is subject to the variable pressures present in the upstream ductwork rather than being precisely matched to the thermal load.
Technical Details and Operation
In a pressure dependent VAV terminal unit, the controller receives a call for heating or cooling from the zone thermostat and repositions the damper accordingly. However, because the unit lacks an integrated airflow sensor, it cannot compensate for static pressure variations in the supply duct. Airflow through the box is governed by the relationship between damper position and inlet pressure, meaning any shift in system pressure causes a proportional change in delivered airflow.
- A change of ±0.5 in. w.g. in duct static pressure at the VAV box inlet can produce airflow variations of ±10% to 20%.
- Control instability is most pronounced at low airflow setpoints, where small pressure changes represent a larger percentage of total flow.
- Typical pressure dependent boxes operate with a single-input (temperature only) control loop, as opposed to the cascaded temperature-to-airflow control loop used in pressure independent designs.
- Without airflow feedback, minimum ventilation airflow rates required by ASHRAE Standard 62.1 cannot be reliably maintained at the terminal unit level.
Applications
Pressure dependent VAV boxes are most commonly found in older commercial buildings or in projects where initial equipment cost is the primary driver. They are sometimes selected for interior zones with relatively stable loads, where pressure fluctuations are less likely to cause noticeable comfort issues. In smaller systems with few terminal units, duct static pressure tends to remain more stable, reducing the practical impact of pressure dependency. However, in large systems serving many zones with diverse load profiles, the interactions between simultaneously modulating boxes create significant pressure swings that amplify the control limitations of pressure dependent units.
Related Standards and Codes
ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) requires that minimum outdoor airflow rates be delivered to occupied zones. Pressure dependent units make compliance verification difficult because actual airflow is unknown without independent measurement. ASHRAE Standard 90.1 (Energy Standard for Commercial Buildings) encourages static pressure reset strategies, which intentionally vary duct pressure to save fan energy but can further degrade the accuracy of pressure dependent terminals. Many building codes referencing these standards effectively favor pressure independent designs for new construction.
Practical Significance
The primary drawback of pressure dependent VAV is inconsistent zone temperature control. Occupants may experience overcooling or undercooling as duct pressures shift throughout the day in response to changing loads across the building. Energy waste is also a concern: zones receiving excess airflow consume more fan energy and conditioning capacity than necessary. Retrofitting pressure dependent boxes with airflow sensors and upgraded DDC controllers can convert them to pressure independent operation, often delivering measurable improvements in comfort and energy performance without full terminal unit replacement.