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Issues such as energy savings and carbon reduction are attracting worldwide attention due to the rapid increase of energy costs and global warming. The main objective of this study was to estimate the power consumption, energy-saving, and the reduction of carbon dioxide (CO2) as greenhouse gas for air-conditioning systems by using two different control strategies for operating the Heating, Ventilating and Air-Conditioning (HVAC) system. These are either a Constant Air Volume (CAV) system (with screw chiller) or a Variable Air Volume (VAV) system (with flooded screw chiller) which has been installed in an office built in Taiwan. Experimental results indicate that the VAV system has greater energy-saving rate than the CAV system. Approximately 46% of the average energy-saving rate was obtained by using VAV system. Furthermore, the power consumption accumulation increased from 102 to 6,111 kWh, and accumulation CO2 reduction changes from 67 to 3,687 kg (The emission factor for electricity = 0.637 kg-CO2/kwh) during office hours from 08:30 (a.m.) to 17:00 (p.m.). Results demonstrate that the energy-saving and CO2 reduction could be significantly enhanced, when the air volume system has been changed from CAV to VAV system. This study is a useful reference for evaluating an indoor environmental policy regarding energy-saving and the reduction of CO2 emissions of air-conditioned offices in subtropical regions.