In certain embodiments, removing non-condensable gas from a cooling system includes trapping contents of a discharge tube of a heat exchanger, where the heat exchanger is in thermal communication with an ambient environment at an ambient temperature. The contents of the discharge tube comprises a vapor portion of a cooling fluid, a liquid portion of the cooling fluid, and a non-condensable gas. The cooling fluid is at a subambient pressure, and the ambient temperature is lower than a boiling point of the cooling fluid. A first additional portion of the cooling fluid is inlet into the discharge tube to increase a pressure within the discharge tube. The vapor portion of the cooling fluid within the discharge tube is allowed to condense. A second additional portion of the cooling fluid is inlet to purge the non-condensable gas from the discharge tube.