Hence, although the mass flow rate of the system with the inclined collector is higher, the air flows more slowly through the collector for a system with a higher roof inclination. For example, the velocity at the collector outlet reaches 6.7 m/s in a zero-slope collector, while it decreases to about 3 m/s in a collector outlet with a height of 5 m. Consequently, there is a significant increase in the heating period of the airflow through the inclined collector. The temperature distributions through the inclined collector are illustrated in Fig. 3. To analyse the effect of increasing the collector-roof inclination on the fluid flow and heat transfer through the system the results of the four simulated cases are compared. The effect of the collector-roof inclination on the mass flow rate is shown in Fig. 4. This figure indicates that when the inlet collector height is fixed and the inclination of the collector roof is increased by increasing the outlet collector height from 1.85 m to 5 m, the mass flow rate increases constantly. Furthermore, it can be noted that for a higher outlet collector height, while the collector-roof inclination is increased, the mass flow rate increases slowly and tends to a constant value.