Variable Air Volume (VAV) Systems

Variable air volume (VAV) systems control the climate of commercial buildings by adjusting the volume of conditioned air delivered to each zone based on actual demand. Rather than supplying a fixed quantity of air regardless of need, VAV systems modulate damper positions zone by zone to maintain temperature setpoints while minimizing energy consumption. The pressure sensor in each VAV terminal unit makes this control possible: it measures the velocity pressure of air moving through the box, converts it to a volume flow rate, and provides the feedback that determines how far the damper opens or closes.

VAV pressure measurement is technically demanding for two reasons. First, at low flow rates, the velocity pressures are very small, often below 25 pascals, and the relationship between velocity pressure and volume flow is nonlinear. A small pressure error at low flow translates into a disproportionately large error in the calculated airflow. Second, each VAV box shares a duct system with every other box in the building, so inaccurate flow readings in one zone affect static pressure and flow balance throughout the system. In a large commercial installation with dozens or hundreds of terminal units, sensor errors compound.

Two sensor capabilities matter most in VAV applications: accuracy at very low differential pressures and long-term stability. VAV terminal units are installed and expected to perform for the life of the building without recalibration. Contamination, temperature cycling, and normal sensor aging all cause zero drift, degrading flow accuracy over time. The HV Series delivers the low-range accuracy VAV systems demand, and the AZ100 auto-zero valve maintains that accuracy by periodically re-zeroing the sensor in place, without system downtime or manual intervention.

Why Choose Superior Sensor for VAV Systems

VAV applications place specific demands on a pressure sensor: low-range accuracy, where flow measurement errors are most consequential; noise rejection in active duct environments; and stability over years of continuous operation. Superior’s NimbleSense architecture was designed to meet all three.

Recommended Sensors

Common Device Features: 3.3V supply
Long-Term Stability is measured after first 12 months
Short-Term Error Band (STEB) is measured over 24 hours, after auto-zero

Common Specifications

VAV Systems FAQ

How does a pressure sensor measure airflow in a VAV system?

VAV terminal units measure airflow with a flow sensor element (typically a Pitot array or cross-flow probe) that converts air velocity into a differential pressure signal. The pressure sensor measures this velocity pressure, and the VAV controller uses a known mathematical relationship between velocity pressure and flow volume to calculate the actual airflow rate. The damper position is then adjusted to align actual flow with the zone’s setpoint. The accuracy of this process depends on the accuracy of the pressure measurement, particularly at low flow rates, where velocity pressures are smallest.

What pressure ranges are typical in VAV terminal units?

Velocity pressures in VAV applications are generally very low, ranging from below 10 pascals at minimum flow to 100-250 pascals at maximum design flow, depending on box size and duct velocity. Duct static pressure, which the air handling unit must maintain to ensure all downstream VAV boxes can reach their maximum flow, typically ranges from 250 Pa to 750 Pa, or higher in larger systems. Because minimum-flow accuracy is critical for demand-controlled ventilation and energy code compliance, the sensor’s performance at the low end of its range is as important as its performance at maximum flow.

Why does sensor accuracy matter more at low flow rates in VAV applications?

The relationship between velocity pressure and airflow volume is governed by the square root of pressure, so errors in pressure measurement are magnified in calculated flow at low pressures. A 5 Pa error at 100 Pa velocity pressure produces a much smaller percentage flow error than the same 5 Pa error at 10 Pa velocity pressure. VAV systems routinely operate at minimum flow during low-occupancy periods, and many demand-controlled ventilation strategies require accurate low-flow measurements to determine when outside air rates can be reduced. A sensor that loses accuracy at low ranges compromises both comfort and energy performance precisely when those conditions occur most frequently.

What causes VAV pressure sensors to drift over time, and how can it be corrected?

Zero drift in VAV pressure sensors stems from several factors: particulate contamination of the sensing ports, temperature cycling that stresses sensor materials, and normal aging of the sensing element. In duct environments, dust and debris accumulate on Pitot probes and sensor ports over time, shifting the effective zero reference. Electrical interference and temperature variations also contribute to drift. The most effective correction is periodic auto-zeroing, in which the sensor ports are momentarily equalized and the zero reference is reset. Superior’s AZ100 auto-zero valve automates this process, correcting accumulated drift without removing the sensor from service or requiring manual recalibration.

How does an inaccurate VAV pressure sensor affect building energy consumption?

When a VAV pressure sensor reads high, the controller detects more airflow than is actually delivered and opens the damper further, delivering excess conditioned air to the zone. When it reads low, the damper closes too much, underserving the zone and causing occupant discomfort, prompting manual override or complaints. Across a building with many VAV boxes, systematic sensor errors cause the air handling unit to maintain a higher duct static pressure than necessary, significantly increasing fan energy consumption. Studies of commercial building energy performance consistently identify VAV sensor calibration as one of the highest-impact factors in HVAC energy waste, making sensor accuracy a direct contributor to operating costs.