AEC was tasked to design a 2N chiller plant for the Defense Information Systems Agency’s (DISA) Data Center. The work included the addition of a fourth chiller and associated pumps to address the increasing data center loads. Significant modifications were made to the plant including re- piping of the plate and frame heat exchangers for waterside economizer / energy savings use and additions to the cooling towers (wind baffles/fan speed control/ drip edges) to allow for wintertime operation. Automated valves were added throughout the piping system to allow for isolation of the chilled water distribution and provide multiple chilled water paths at all times. The Siemens Building Automation System (BAS) system was modified to include the new chiller, the additional control valves and the new sequence for waterside economizer. All work was designed to maintain the fully operational cooling system at all times during the construction. The project also included the replacement design of computer room air handlers and modifications to the BAS to measure temperature and humidity levels throughout the center to coordinate and control the air handlers and existing humidifiers. The work required structural and architectural design of new wall penetrations, supports, interior and exterior housekeeping pads.