The dynamic performance of a discharge air control system with a proportional or proportional plus integral controller is modeled and analyzed. The model includes a sensor, controller, actuator/valve, and heating coil. The mathematical model of the heating coil is non-linear since it requires the time constant to be a function of the water flow rate. The solution of the mathematical model was obtained with the fourth-order predictor-corrector numerical integration method. System performance is analyzed as a function of the following parameters: proportional gain, integral gain, actuator with and without positioner, and with constant and variable valve-coil gain.