TABLE OF CONTENTS HVAC systems are sized to satisfy a set of design conditions, which are selected to generate a maximum load. Because these design conditions prevail during only a few hours each year, the HVAC equipment must operate most of the time at less than rated capacity. The function of the control system is to adjust the equipment capacity to match the load. Automatic control, as opposed to manual control, is preferable for both accuracy and economics; the human as a controller is not always accurate and is expensive. A properly designed, operated, and maintained automatic control system is accurate and will provide economical operation of the HVAC system. Unfortunately, not all control systems are properly designed, operated, and maintained.
The purpose of this chapter is to discuss control fundamentals and applications in a concise and understandable manner. For a yet more detailed discussion, see the references at the end of the chapter. The diagrams shown are generic and use symbols defined in Fig. 8.65 at the end of the chapter.
Control systems for HVAC do not operate in a vacuum. For any air conditioning application, first, it is necessary to have a building suitable for the process or comfort requirements. The best HVAC system cannot overcome inherent deficiencies in the building. Second, the HVAC system must be properly designed to satisfy the process or comfort requirements. Only when these criteria have been satisfied can a suitable control system be implemented.
