INTRODUCTION

In order to discuss the maintenance and operations of system control equipment, it is important to understand their purpose. Fundamentally, a control system does four things: (1) establishes a final condition, (2) provides safe operation of the equipment, (3) eliminates the need for ongoing human attention, and (4) assures economical operation.

Maintaining a set final condition is often considered the only purpose of the system because it has a direct impact on occupants of the facility. The comfort level of each individual is the end product of all control efforts, yet ASHRAE design standards are set to satisfy only 90 percent of all occupants. Well-designed systems that are operated correctly and maintained to their original delivered condition are capable of doing much better than this.

Safety pertains to both individuals and property. The most obvious safety-related HVAC controls are pressure and temperature cutout switches that shut off equipment to prevent damage to both the device and the operator. It is interesting to note that levels of safety consciousness vary throughout the world. While the general pattern would seem to indicate more concern for safety in economically developed countries, there are variances from country to country based on culture and operational practices. For example, it is quite normal for a motor control system installed in Germany to monitor each position of a hand-off-auto switch, as well as all electrical overload devices. As we develop into a completely global society based...