Heating, ventilating, and air conditioning (HVAC) systems are among the most complex of all building service systems. To maintain a high level of occupant comfort, worker productivity, indoor air quality (IAQ) and energy efficiency, HVAC systems must be properly designed, installed, maintained and operated. Inherent in the design intent is that the system operates with energy efficiently and maintains acceptable IAQ in the building. The verification of performance tests for fans and pumps include pressure tests across the fan or pump, pressure tests in the air or water distribution system, and flow readings at the air or water terminal. Performance testing a chiller or cooling tower involves a heat balance. System commissioning is the process of assessing system performance to assure proper functioning and adherence to design criteria, including the actual start-up and testing of the HVAC system. It is generally assumed that the HVAC systems and lighting systems account for most of a building’s energy use.