The present paper details the outcomes of a numerical analysis performed to study the influence of channel's height (channel's span wise aspect ratio) over the heat transfer rates of an upward solar air heater. Upward solar air heater is modelled and simulated using the commercially available ANSYS 19.2 Fluent software. The major objective of the present study is to highlight the effect of channel's height on heat transfer rate in upward solar air heater. Solar air heater employs an aluminium flat plate having 0.90 m length × 0.75 m width. The depth of the collector considered are 150, 100, 75 mm having aspect ratios of 5, 7.5, 10 respectively. Moreover, the following process variables such as mass flow rates (0.004 to 0.020 kg/s in steps of 0.002) and aspect ratios (5, 7.5,10) are considered in order to understand the performance of the upward solar air heater. From the analysis it is found that the average value of Nusselt number increases with increase in flow rate of air, but the exit air temperature decreases as the flow rate increases. The rise in flow rate of air from 0.004 to 0.02 kg/s caused Nusselt number to increase from 33.65 to 69.55 (aspect ratio 10), which indicates that upward solar air heater performs effectively at higher mass flow rates. The obtained results are useful to design and test solar air heaters.