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In this paper, microscopic and macroscopic flow analyses have been employed to identify and analyze the causes of discrepancies of power law zonal models. The analyses show that the use of constant flow coefficient (Kf ) is one of the reasons for the discrepancy. Computational Fluid Dynamics (CFD) was employed to estimate Kf for isothermal condition. The variations of Kf as function of room height were investigated for two types and locations of diffusers. Five levels of Reynolds numbers (Re) were used. The flow coefficients for horizontal (Kfx) and vertical (Kfy) flow were estimated separately. The result has shown that Kfx and Kfy vary differently and can be affected by variability of the flow field, which can vary depending on the choice of inlet device and relative location of inlet. Implementation of the zonal model which employs a variable flow coefficient has shown a significant improvement compared to the existing zonal model.