Impacts of Static Pressure Reset on VAV System Air Leakage, Fan Power and Thermal Energy ───Part I: Theoretical Model and Simulation

As for a variable air volume (VAV) system, the air duct static pressure is a typical control variable maintained by modulating supply fan speed. The static pressure equals to the summation of the duct pressure loss downstream of the sensor to the terminal box and box inlet static pressure. Typically, the air duct static pressure is set as a constant set point based on the system design information and sensor location. However, under partial load conditions, the terminal box dampers have to be closed more since the required airflow is less than the design airflow which directly leads to much less pressure loss. Thus the static pressure set point should be reset lower in order to reduce fan power, avoid noise at terminal box dampers and box damper malfunction due to excessive pressure. Different static pressure reset schedules are reviewed and compared, considering the influence of outside air temperature on the building load, availability of the VAV box damper positions, the airflow ratio based static pressure reset has also applicable advantages over the existing constant static pressure set point and two typical reset methods. This paper presents the theoretical models to express the impacts of static pressure reset on fan airflow, fan head, air leakage, fan power and thermal energy for both pressure independent and pressure dependent boxes. The impacts are also demonstrated using the parametric analysis and numerical results to show the benefits of the static pressure reset including reducing fan power, cooling energy and heating energy.