The aim of this study was to develop a simplified CFD model for the inlet jet of a swirl diffuser for the simulation of room airflow patterns. The swirl diffuser creates a complex flow pattern with high induction of room air, thus possessing a challenge for simulation. The studied diffuser was a model intended to be used in large enclosures. The flow pattern was adjustable between two basic modes: radial swirl jet for cooling conditions and compact downward swirl jet for heating conditions. For developing the CFD model, a series of laboratory measurements of the flow field was carried out. The boundary conditions for the diffuser were determined from the near field velocity measurements close to the diffuser surface. For validating the model, velocity distributions were measured at different distances from the diffuser. The aim was to develop a single model that can be used for both heating and cooling adjustments. The dimensions of the model were designed to follow approximately the geometry of the diffuser. The boundary conditions were calculated from the measured momentum and mass flow rates from the diffuser openings. The estimated momentum loss between the diffuser and the nearest measurement plane was added to the inlet momentum. The developed model was validated in isothermal conditions for both adjustments and for non-isothermal conditions for the heating mode. The model was found to predict well the flow patterns and velocity levels in both modes. Two turbulence models were used in the simulations, k-ε and SST. The differences between the results of the two models were small.