Air conditioning, or HVAC&R, is an active, rapidly developing technology. It is closely related to the living standard of the people and to the outdoor environment, such as through ozone depletion and global warming. Currently, air conditioning consumes about one-sixth of the annual national energy use in the United States. At the beginning of a new millennium, in addition to the publication of ASHRAE Standard 90.1-1999 and ASHRAE Standard 62-1999, often called the Energy standard and Indoor Air Quality standard, the second edition of Handbook of Air Conditioning and Refrigeration is intended to summarize the following advances, developments, and valuable experience in HVAC&R technology as they pertain to the design and effective, energy-efficient operation of HVAC&R systems: First, to solve the primary problems that exist in HVAC&R, improve indoor air quality through minimum ventilation control by means of CO2-based demand-controlled or mixed plenum controlled ventilation, toxic gas adsorption and chemisorption, medium- and high-efficiency filtration, and damp surface prevention along conditioned air passages. ANSI/ASHRAE Standard 52.2-1999 uses 16 minimum efficiency reporting values (MERVs) to select air filters based on particle-size composite efficiency. Energy conservation is a key factor in mitigating the global warming effect. Electric deregulation and the use of real-time pricing instead of the time-of-use rate structure in the United States have a significant impact on the energy cost. ANSI/ASHRAE Standard 90.1-1999 has accumulated valuable HVAC&R energy-efficient experiences since the publication of Standard 90.1-1989 and during the discussions of the two public reviews. For buildings of one or two stories when the outdoor wind speed is normal or less than normal, the space or building pressurization depends mainly on the air balance of the HVAC&R system and on the leakiness of the building. A proper space pressurization helps to provide a desirable indoor environment. Second, there is a need for a well-designed and -maintained microprocessor-based energy management and control system for medium-size or large projects with generic controls in graphical display, monitoring, trending, totalization, scheduling, alarming, and numerous specific functional controls to perform HVAC&R operations in air, water, heating, and refrigeration systems. HVAC&R operations must be controlled because the load and outside weather vary. The sequence of operations comprises basic HVAC&R operations and controls. In the second edition, the sequence of operations of zone temperature control of a single-zone VAV system, a VAV reheat system, a dual-duct VAV system, a fan-powered VAV system, and a four-pipe fan-coil system is analyzed. Also the sequence of operations of a plant-building loop water system control, the discharge air temperature control, and duct static pressure control in an air-handling unit are discussed. Third, new and updated advanced technology improvements include • Artificial intelligence, such as fuzzy logic, artificial neural networks, and expert systems, is widely used in microprocessor-based controllers. • BACnet is an open protocol in control that enables system components from different vendors to be connected to a single control system to maximize efficiency at lowest cost. • Computational fluid dynamics is becoming an important simulation technology in airflow, space diffusion, clean rooms, and heat-transfer developments.