In this study, the effects on energy conservation and indoor environment are estimated with three different air-conditioning and heat source system operation or design options; namely, 1) changing the air-conditioning setting temperature from present condition; 2) optimizing the heat source equipment capacity; 3) changing the heat source equipment combination. These options are evaluated through the use of BEMS data taken from an office building. Firstly, the temporal tendency of internal heat load and outdoor air load under practical operation are analyzed by BEMS data. Secondly, by using the heat gain schedules, air-conditioning load is calculated to be used as a standard value for comparison. Simultaneously, air-conditioning loads are calculated after changing the three operation and design factors specific above. Finally, primary energy consumption for each case is calculated. Conclusively, this study shows that optimizing heat source equipment capacity and combination can promote energy conservation while maintaining the indoor environmental quality.