Data center cooling can constitute a significant portion of the total data center energy usage, with typical cooling energy expenditures approximately 50% of the IT energy use. Much of this energy consumption occurs at the refrigeration/chiller plant and in the Computer Room Air Handlers (CRAH) that cool and condition the air used to cool the electronics racks. To reduce cooling energy use, a data center test facility was designed and constructed to reduce cooling energy use to less than 5% of the total IT energy use through a combination of warm water cooling of the electronics and liquid-side economization. Several data center operating conditions, such as changes in liquid and air flow rates, heat exchanger arrangements and addition of propylene glycol were investigated to determine their impact on the energy consumption and thermal performance of the key cooling equipment. Day long runs collected from summer and fall days are also reported to illustrate the impact of external weather conditions and loop operating conditions on the thermal performance and energy consumption of the dual-loop data center test facility. The work presented highlights the impact of various operating conditions in influencing the cooling energy use and improving data center energy efficiency in chiller-less, ambient-air cooled data center designs using water cooled servers. © 2012 IEEE.