The present study aimed to develop a computational model to understand the effect of air gap depth on the Trombe wall system. The simulation was performed for midday of January 17, 2017, at a Kirkuk city/Iraq; at this time, the solar intensity was at a maximum value equal to 487.1 W/m2, ambient temperature 10.1°C, and wind speed 0.7 km/h. The result of the simulations is investigated with the experimental work in the literature. The various parameters such as thermal efficiency, inlet and outlet temperatures from air gap, room temperature, and air mass flow rate in the air gap channel have been considered in simulation. The comparison result showed a good agreement between the predicted results and experimental work. This research work will be useful for the research community to understand the effect of air gap depth in Trombe wall system.