The main objective of this on-site study is to use a full-scale Heating, Ventilating, and Air-Conditioning (HVAC) system installed in an office building in Taiwan for comparing the power consumption, energy-saving, and carbon dioxide (CO2) reduction of two different strategies for controlling the HVAC. These strategies are the Constant Volume (CV) system [Constant Air Volume+Constant-flow], and the Variable Volume (VV) system [Variable Air Volume +Variable-flow]. The on-site experimental results indicate that average power consumptions are 164 kW for the CV system, and 88 kW for the VV system; the average electric current drops from 469 A for the CV system to 258 A for the VV system. Approximately 46% of the average energy-saving can be achieved if the HVAC system is operated as a VV system. Additionally, the reduced quantity of accumulated CO2 emission varies from 67 to 3687 kg with 0.637 kg CO2 kwh(-1) emission factor during the office hours of 08: 30 (a.m.)-17:00 (p.m.). The results demonstrate that switching the operation of an office building HVAC system from CV to VV will significantly enhance energy-savings and CO2 reduction. This studywill offer useful information for evaluating an indoor environmental policy with respect to energy-savings and CO2 emission reduction for office HVACs used in subtropical regions. Copyright (C) 2010 John Wiley & Sons, Ltd.