A zonal model with two different approaches for expressing mass flow rates (the conventional method based on relations for flow in conduits and the shear stress QSNA-Newton's approximation) is used to predict airflow patterns in a ventilated isothermal enclosure. A new proposed 3D wall jet model gives much better agreement with experimental results than the previously available 2D wall jet model. The outlet boundary condition (((m)over dot)(out) = ((m)over dot)(in), or P-out = P-atm) has no influence on the mass flow rates. However, the pressure condition should be chosen when there is more than one outlet. Based on the calculated results, it is recommended that the shear stress approximation should be privileged over the conventional method since the latter predicts completely unrealistic average cell pressures with both outlet boundary conditions. (C) 2007 Elsevier Ltd. All rights reserved.