Kaustubh Phalak is a doctoral student in Department of Civil, Architectural and Environmental Engineering, University of Miami, Coral Gables, FL. Dr. Gang Wang is assistant professor in Department of Civil, Architectural and Environmental Engineering, University of Miami, Coral Gables, FL. Jose Varona is Associate Director for Energy Management Systems at University of Miami Coral Gables, FL. Improving Valve Operation Using Cascade Control in Single Zone Air Handling Units Kaustubh Phalak Gang Wang, Ph.D., PE Jose Varona Student Member ASHRAE Member ASHRAE ABSTRACT Single zone (SZ) air handling units (AHUs) are widely applied in the conditioned spaces. A SZ AHU typically consists of a chilled water cooling coil, a hot water heating coil, and a supply fan. For a constant volume (CV) SZ AHU or variable air volume (VAV) SZ AHU operating at a minimum airflow, the control valve of either the cooling coil or heating coil is modulated to vary the supply air temperature and consequently control space air temperature. Traditionally, a single control loop is applied to modulate the control valve directly based on the space air temperature. The traditional control is simplistic in nature, however, suffers significant drawbacks. Due to the thermal capacity of both the water in the coils and the air in the conditioned space, the system often becomes unstable due to nonlinear characteristics of the SZ AHU system and leads to hunting of the control valve. On the other hand, cascade control makes the control system more adaptive and robust. A cascade control can be applied to a SZ AHU in order to stabilize the system. The primary controller reads the room air temperature and determines the required supply air temperature for a secondary controller which then controls the heating/cooling coil valve. The purpose of this paper is to demonstrate the stability of the cascade control method in a SZ AHU system. A model of the SZ AHU system with transfer functions is developed for conventional and cascade control methods and root-locus analysis is performed. The results conclude that the cascade control improves the stability of the control valve by reducing the sensitivity to the change in the operating conditions. INTRODUCTION A single zone (SZ) air handling unit (AHU) system is a type of centralized HVAC systems in which the heating or cooling operation in AHUs is controlled by a single thermostat located in the space. This type of system is widely used in residential and small commercial buildings due to simple design and low cost. A SZ AHU is an effective