This paper describes an HVAC (heating, ventilating and air conditioning) system controller which employs a control algorithm using either fuzzy logic reasoning or rough set theory. The controller deduces the appropriate control outputs from sensor readings. The system is capable of controlling temperature and humidity. To maintain temperature at the reference point, the controller adjusts the flow of hot water into a heating coil for heating operation and the flow of chilled air through an air duct for cooling operation. To control humidity, the controller turns on and off a humidifier.