The outlet air temperature of a solar collector would be required to be as high as 50℃ for heating rooms and hot water supply in the winter time and as low as 80℃ in the summer time for protecting the fan from the heat-resistant problem. In this study, the outlet air temperature of a collector was tried to be constant by adjusting the flow rate of the air by means of using an inverter to regulate the power for the fan. A non-steady state analysis on the thermal performance of flat plate collectors was also made for aiming to apply the analysis to simulation of the performance of collectors for various conditions. It was found that the outlet temperature of the collector was stable and was kept constant during the daytime and that the analysis would be useful for simulating the performance.