A detailed study has been carried out to clarify the conditions of boiling incipience phenomena for natural flow in vertical tube thermosiphon reboiler. A semi-empirical model has been proposed that considers the effect of turbulent eddies and submergence for the data available in literature. Boiling incipience in liquid films is influenced both by turbulent eddies and submergence, and their effect is dominant with increasing heat flux. The superheat for boiling incipience was determined experimentally for different types of organic liquids including water, covering a wide range of physical properties. The predicted results from the proposed model and experimental data available in literature show a consistency for all fluids investigated in this study. It has an average absolute relative error of 15% for the proposed unified correlation of all fluid systems together, which have a wide range of thermophysical properties.