"INTRODUCTION The objectives of this study are to develop and validate a dynamic air handling unit (AHU) model. The development process of the dynamic system model (referred to as 1312 model hereafter) that includes a single duct dual fan variable air volume (VAV) AHU system and four building zones served by the VAV AHU system are introduced in the companion paper (Li and Wen 2009). The 1312 model is developed in HVACSIM+ environment and is based on the E51 model developed from ASHRAE research project RP825 (Norford and Haves 1997). The test facility used for this study is also described in Li and Wen (2009). In this paper@ the validation strategy@ process@ and final results for 1312 model are described. Validation of Heating Ventilating@ and Air Conditioning (HVAC) and building zone simulation dynamic models is not a trivial issue. Detailed review about simulation code verification and validation is provided by Reddy et al. (2006). While verification deals with determining whether the equations are solved correctly@ validation involves solving the right Equations and comparing simulation results against field or experimental data. There are publications in the literature that discuss HVAC system dynamic model verification and validation such as those focus on (a) component models (Clark et al. 1985@ Zhou and Braun 2007); (b) primary systems (Henze et al. 1997@ Wang et al. 2000); and (c) air conditioning process and its interaction with building zones (Brandemuehl et al. 1990@ Ahmed et al. 1998). Two papers were found that specifically discussed AHU dynamic model validation: ? Chen and Deng (2006) developed a dynamic simulation model for a direct expansion VAV air conditioning system consisted of a VAV air distribution subsystem and a DX refrigeration plant. AHU model was part of the overall model. A test rig was developed for validating the model. However@ the validation process only included comparing model outputs and experimental data under an open loop step change of compressor speed (one speed adjustment). ? No real weather conditions or internal loads was applied to the model. Nassif et al. (2008) developed a series of simplified component models for an AHU@ a VAV terminal unit@ building zone@ and their control systems. Real operation data collected from the system control system were used to obtain model parameters. Model outputs were compared with system measurements. However@ the component models were not connected to each other and it was unknown how well the entire system model would perform if all component models were connected. The above literature review indicates that there is a lack of a comprehensive validation study for AHU dynamic models that compares the entire system model predictions with real operation data. In this study@ three perspectives of the 1312 AHU model are to be validated: 1. Parameters: during the model development process@ all parameters are obtained from either nominal design values or from manufacturer catalogs. Those values often do not reflect the true parameters for a real system. An important part of the validation process is to first ""tune/ calibrate"" the parameters in the simulation model from system measurements; 2. component models: component models used in HVACSIM+ may not be able to simulate the test facility AHU performance satisfactorily because HVACSIM+ component models generally represent new and ideal component behaviors; and 3. system performance: even after all component models perform satisfactorily@ the system performance may still not be satisfactory due to error propagation and numerical calculation stability."