This paper presents and discusses the experimental measurements versus the results from the available correlations in the literature that are commonly used to predict the free convection heat transfer coefficient between the surfaces and the flowing air. The experimental investigations were accomplished using a rectangular duct comprising a flat plate-glass cover as the solar air heater with the following dimensions: 0.48-m width x 0.07-m depth x 2-m length. The absorber plate was made of 1-mm gauge of aluminium. The measurements were performed at inclination angles of 30 degrees, 50 degrees, and 70 degrees to determine the optimum angle of the absorption-free convection mechanism in a solar air heater. The comparison is presented and discussed in terms of the Nusselt number. The optimum inclination angle to achieve the best collector performance was found to be 50 degrees. The results of the present work can assist in resolving the issue of predicting the free convection in thermal passages. (C) 2014 Elsevier Ltd. All rights reserved.