Buildings and their occupants generate a large amount of carbon emissions. In Korea, buildings contribute to about 30% of the total greenhouse gases emissions, and the proportion has been rapidly increasing to the level of the developed counties (i.e., more than 40% of the total emissions. A direct expansion air handling unit, of which a refrigerant is directly delivered to the heating and cooling, has a potential to save cooling and heating energy use, compared to water-based central air conditioning systems. The aim of this study is to compare heating and cooling energy uses of an identical office building but with different air conditioning systems, i.e. direct expansion and water-based air conditioning systems. Dynamic building energy simulations that reflect the actual use of a monitored building and its air handling unit operation have been conducted in this study. Simulation results show good agreement with the actual energy consumption obtained from the field measurements of the building. Our study quantifies the amount of cooling and heating energy uses saved by a direct expansion air handing unit and reveals reasons for this savings, i.e. higher energy efficiency of the unit and reduction in pump and fan energy demands. (C) 2014 Elsevier B.V. All rights reserved.