In recent years, net zero-energy buildings (ZEB) have become essential to reduce greenhouse-gas emissions. The purpose of this study was to verify the energy-saving performance of air-conditioning and hot-water supply systems in an office building. The target building adopted a system that used water circulating within the building as a heat source for air conditioning and the hot-water supply. To determine the energy-saving performance of the air-conditioning system, we measured the flow rate and temperature of the heat-source water and the system power of the water heat-source unit (WHSU), and the efficiency of the WHSU was calculated. It was found that the WHSU exhibited the anticipated cooling efficiency in areas with a larger heat load, whereas the efficiency was lower than the rated value during the heating period. The following year, the flow rate of the heat-source water was adjusted, focusing on improving the energy efficiency of the entire system. As a result, the circulation-pump power was significantly reduced, whereas the efficiency of the individual WHSU was lower than that of the previous year. In addition, the coefficient of performance of the new hot-water heat-pump system was found to be 2.8 on average.