A theoretical model is formulated, utilizing an integral technique, to describe the thermal boundary layer development. Special case closed-form solutions are obtained for 0.72 less than or equal to Pr less than or equal to 10 to predict the local and average heat transfer coefficient for combined forced and natural convection from an isothermal vertical flat plate, for both assisting and opposing flows. No opposing flow closed-form solutions are known to exist in the current literature. Assisting and opposing flow experiments were performed to measure the average heat transfer coefficient with air for two flat plate heat transfer models of different lengths. The predictive capability of the present theoretical model was compared to this experimental data with excellent agreement. Excellent agreements also found to exist with the experimental data and numerical solutions of other researchers.