The growth of thick epitaxial 4H-SiC layers with low defect density is an essential step for the fabrication of SiC based devices. Cold- and hot wall reactors using silane and propane diluted in hydrogen were used in this study. The typical growth temperature range is 1700–1900 K and total pressure range 10–100 kPa. The resulting epilayers exhibit low background doping, low defect density and good thickness uniformity. The main problem is that it is difficult with this first generation of reactors to ensure a constant temperature over large wafer. A 3D simulation approach of heat and mass transfer was used with three objectives. The first one is to have a visualization of the flow, temperature and gaseous species fields in the standard reactor. The second one is to propose solutions for the optimal control of the temperature field and the subsequent uniformity of the epilayers over large dimensions. The third one is to improve the kinetic databases in this temperature range which has been very little investigated.