A control system that prevents air from entering an air-handling unit through the exhaust air.

Traditional air-handling unit (AHU) control systems link the position of the exhaust air damper, recirculation air damper, and outdoor air damper. Tests at the National Institute of Standards and Technology (NIST) on a variable-air-volume (VAV) AHU have shown that air can enter the AHU through the exhaust air damper. This can negatively impact indoor air quality if the exhaust air duct is located near a pollution source. This paper presents a new control system for variable air volume AHU's that use volume matching to control the return fan. The new control system links only the position of the exhaust air damper and recirculation air damper. During occupied times, the outdoor air damper is in the fully open position. Simulation and laboratory results are presented to compare the new control system and a traditional control system. Several cases are simulated to examine the effect of damper sizing and system load on airflow in AHU's. The simulations demonstrate that the new control system can prevent air fiom entering the AHU through the exhaust air damper for conditions that the traditional control system cannot. A case demonstrating the limits of the new control system to prevent this phenomenon is included in the simulation results. The laboratory results provide further evidence that the new control system prevents air from entering the AHU through the exhaust air damper for conditions that cause the phenomenon with the traditional control system.