Indoor heating systems currently used are highly dependent on fossil fuels; hence, it is urgent to develop a new type of heating system to achieve carbon zero emission. A Solar air heater is eco-friendly because it generates nearly zero greenhouse gases. In this study, a parametric study was conducted for optimizing solar air heater design applicable to indoor heating. The thermo-fluid dynamical characteristic of the solar air heater was analyzed by comparing the velocity of the air flow through the passage and the average exit temperature from the heater having different internal flow path structures. The average exit temperature can be increased by forming some path inside the heater. An additional case study was carried out to optimize the ideal quantity of phase change material (PCM) in terms of mass fraction and heat capacity for various operating conditions. Excessive amount of PCM deteriorates the performance of the air heater unless the entire PCM could be melted during the daytime. The solar air heater can fully replace or partly assist a conventional heater for indoor heating, and it could reduce approximately 0.6 tCO2 per year. These results are expected to provide useful information on the optimal design of solar air heaters.