The main contribution of this study is to highlight the position effect of the absorber plate as a passive heat transfer enhancement technique on the ventilation and thermal performance of a solar air heater, including outlet air temperature, airflow rate, and thermal efficiency. The turbulent free convection heat transfer in a vertical flat-plate solar air heater with single and double passes is numerically studied. A total of four cases of airflow (three double-pass and one single-pass air heater) are simulated and compared. The set of transient governing equations, including the continuity, momentum, and energy are solved to simulate the turbulent free convection airflow inside the heater based on the  turbulence model, while the transient conduction equation is solved inside the solid elements, i.e., bottom plat, absorber, and glass cover based on the finite-element method. The results reveal that the position of the absorber plate has a double-edged sword impact on the performance of solar air heaters, including the airflow rate and outlet air temperature. Moreover, a 100% increase in the natural air flow rate is observed for the double-pass heater compared to the single-pass one. Moreover, a 15% improvement in thermal efficiency can be obtained from 68% to 78% in the double-pass solar air heater based on the position of the absorber plate. Based on the criteria, including the outlet temperature, airflow rate, or thermal efficiency, the presented results can be used as a good reference to choose an alternative for solar air heaters.