Fans, motors, coils, and other control components enable a heating, ventilating, and air-conditioning (HVAC) system to function smoothly. An explanation of these control components and how they make school HVAC systems work is provided. Different systems may be compared by counting the number of controlled devices that are required. Control systems are categorized according to their means of data processing, transmission, and power source: pneumatic, electric, electronics, direct digital, and self-powered. Likewise, the HVAC system is composed of various components: sensors, controllers, actuators (the system's power elements), and controlled devices, (e.g., dampers, valves, and speed regulators). All of these control elements work together to form a control loop. An overview of HVAC systems controls is offered, which focuses on unit ventilators, variable air volume systems, rooftop air conditioning, and supplemental systems. Monitoring the HVAC is also important, and methods such as multiple location supervision are explored. It is suggested that control system maintenance is another important component of any HVAC system, which entails the inventorying of tools and parts. Some control problems that may affect IAQ are provided. (Contains four figures, six references, and a glossary of terms.) (RJM)