There are many panel radiator manufacturers in Turkey. Their panel radiator designs are very similar and radiators that have been manufactured by various producers have very similar thermal efficiency values and specific heating per unit weight of the radiator. In this study, CFD analysis of existing panel radiators were made with a commercial CFD code of STAR-CCM+ with variable connection methods in three-dimensional space. Numerical thermal efficiency values were obtained and were compared with given catalogue values. Panel-convector-convector-panel, Type-22-600x500 steel panel radiator was used in this numerical study. In the content of this analysis, the effect of variation at air-side convective heat transfer coefficient on the thermal output was also investigated. After optimum air-side convective heat transfer coefficient had been determined, numerical analyses of the panel radiator were done based on this value. For top-bottom-opposite-end connection, mass flow rate distribution is greater than exact flow at first two and last three vertical ducts. In the middle of the vertical ducts towards inlet boundary, mass flow rate distribution corresponds to about zero. For top-bottom-same-end connection, mass flow rate distribution is greater than exact flow at first four vertical ducts. In the last part of the vertical ducts, mass flow rate distribution corresponds to about zero. Numerical results and commercial catalog values are very close each other for TBOE and TBSE connections, and thus two basic connection methods can be used for this panel radiator.