This chapter reviews the basic operation of a general Heat, Ventilation and Air Conditioning (HVAC) system and then focuses on Computer Room Air Conditioners (CRACs).

5.1 General Overview of an Air-Conditioning System Building envelopes, data centers, and telco/telecom rooms (e.g., telecommunications central switch offices) are sealed, and an HVAC system controls the air exchange and conditions the air that is delivered to the closed environment. This is called mechanical ventilation.* The HVAC system brings outdoor air into a building, humidifies or dehumidifies it, and heats or cools it to meet the ventilation and thermal comfort needs of the occupants (and/or the cooling needs of equipment, such as IT/networking equipment), and to dilute the contaminants inside the building. CRACs are optimized for data center/telecom site applications. Below we discuss some general principles about HVAC operation and the focus on CRACs.