System and method are provided for cooling an electronics rack. A modular cooling unit (MCU) is associated with the rack to provide system coolant to an electronics subsystem and a bulk power assembly. The MCU includes a liquid-to-liquid heat exchanger, and defines portions of facility and system coolant loops. Chilled coolant from a facility source is passed through the liquid-to-liquid heat exchanger to cool system coolant flowing through the system coolant loop. The system also includes an air-to-liquid heat exchanger in fluid communication with the system coolant loop, a pump in fluid communication with the system coolant loop, and a controller. The controller controls operation of the pump to adjust flow of system coolant through the system coolant loop dependent upon a mode of operation. In a standby mode, system coolant flows through the air-to-liquid heat exchanger at a lower flow rate, and expels heat to ambient air.