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This study investigates the thermal load features in office buildings and proposes an innovative Integrated Air Handling Unit (IAHU) concept in order to achieve energy savings with conventional office building air handling systems. The corresponding deduction of IAHU for an acceptable Indoor Air Quality (IAQ) and better energy performance is conducted. The system variables and constraints are analyzed in detail to understand the feasibility and operability of IAHU. The control logics and implementation methods are elaborated for typical system layouts. With an IAHU operation, the internal heat gain can be transferred from an interior region into an exterior region in winter. The sensible load and latent coil load can also be decoupled in mild weather. To evaluate the performance of IAHU for buildings, especially insufficiently sub-metered buildings, a simplified simulation method is proposed. The theoretical modeling process is provided. Through a case building simulation, it is found that, by converting a Two Air Handling Unit (TAHU) system into an IAHU system, about 14% of thermal energy can be saved for the case building, which is equivalent to a 3.5 MBTU/ft2 yr saving in the given climate. By transferring the internal heat gain from the interior region to the exterior region, 58% of the total savings, by applying IAHU, can be achieved in winter time and 17% in swing seasons. Another 25% savings comes from the sensible and latent coil load decoupling of using IAHU in summer mild weather.