Heat pumps have been spotlighted as efficient building energy systems because they have great potentials for energy reduction in building air conditioning and reducing CO2 emission. In this study, a multi-function heat pump which has the functions of heating, cooling, and hot water supply was designed and its performance was investigated according to operating modes. In the cooling-hot water mode, the capacity and COP were enhanced as compared to other modes because the waste heat from the outdoor heat exchanger was utilized as useful heat in the indoor heat exchanger. In the heating and hot water supply mode, the compressor speed should be increased to get appropriate heating and hot water capacities. For all operating modes, the system could be optimized by adjusting the superheat. (C) 2012 Elsevier Ltd. All rights reserved.