A coolant control unit for a liquid cooled electronics system is provided, which includes an external coolant inlet and outlet for receiving and returning external coolant; an internal coolant loop for circulating coolant to the electronics system; a first and second control valve coupling the external coolant inlet and outlet to the internal coolant loop; a heat exchanger connected between the first and second control valves; and control logic for controlling operation of the coolant control unit in one of an external coolant mode and an internal coolant mode. In the external coolant mode, the first and second control valves allow passage of external coolant through the internal coolant loop to the electronics system, and in the internal coolant mode, the first and second control valves isolate coolant within the internal coolant loop from the external coolant inlet and outlet, and pass the coolant therein through the heat exchanger.