Abstract <p>Optimizing the air gap between the absorber and the glass cover is a critical factor in designing solar air heaters. This study aims to analyze the effect of gap variation on a double pass solar air heater’s (DPSAH) performance through four distances 3, 6, 9 and 12 cm, using experimental data and numerical analysis via CFD (ANSYS Fluent). The numerical model was validated with experimental results in good agreement. The findings indicate that increasing the height of the upper channel correlates with the enlargement of a vortex, intensifying conduction over convection mode at the onset of this channel, and reducing heat loss by eliminating the stagnation vortex in the lower corner, the maximum local heat transfer coefficient was observed at the lower channel by 14.61 W/m² K. The thermal efficiency reached 66.4% in 12 cm, dropping by 2.4, 7.8 and 14.4% for 9, 6 and 3 cm configurations, respectively. Similar to the exergy efficiency, which ranges between 2.1 and 2.6%. Furthermore, the optimum configuration (12 cm) was tested to a variable mass flow rate of 0.006, 0.012, and 0.015 kg/s, with the highest rate yielding a thermal efficiency of 72.5%.</p>