What is the role of pressure sensors in duct static pressure reset strategies?

Duct static pressure sensors provide the input signal for fan speed control in VAV systems, and ASHRAE Guideline 36 specifies that the static pressure setpoint should be continuously reset based on the damper position feedback from all VAV boxes serving the system. The sensor should be installed in a straight duct section, 2/3 to 3/4 down the main duct, and must use a pitot tube or static pressure probe oriented perpendicular to airflow to avoid velocity pressure contamination. The reset algorithm works by polling all VAV damper positions every 2–5 minutes and reducing the duct static pressure setpoint in small increments (0.1–0.2 in. w.g.) until at least one VAV box reaches 90–95% open, ensuring that the fan produces only as much pressure as the most-demanding zone needs. This strategy can reduce fan energy by 30–50% compared to a fixed static pressure setpoint, particularly during part-load conditions that dominate most operating hours.