We report the energy and exergy efficiencies of Aquasar, the first hot water cooled supercomputer prototype. The prototype also has an air cooled part to help compare the coolants's performances. For example, a chip/coolant temperature differential of only 15 °C was sufficient for chip cooling using water. The air cooled side, however, required air pre-cooling down to 23 °C and a chip/coolant temperature differential of 35 °C. Whereas extra exergy was expended for air pre-cooling, the higher thermal conductivity and specific heat capacity of water enabled coolant temperatures to be safely raised to 60 °C. Using such hot water not only eliminated the need for chillers, it also opened up the possibility of heat reuse. The latter was realized by using the hot water from Aquasar for building heating. A heat recovery efficiency of 80% and an exergetic efficiency of 34% were achieved with a water temperature of 60 °C. All these results establish hot water as a better coolant compared to air. A novel concept of economic value of heat was introduced to evaluate different reuse strategies such as space heating and refrigeration using adsorption chillers. It was shown that space heating offers the highest economic value for the heat recovered from data centers. © 2012 Elsevier Ltd.