The requirements of different standards for the temperature of the external surfaces of pyrolysis furnace walls in petrochemical plants and the real values are reported. A different approach to determining the boundary conditions in thermal calculation of the furnace lining is noted, which makes it difficult to evaluate operation and select lining material. The outer wall temperature of pyrolysis furnaces with linings of different design was analyzed. A conclusion was drawn concerning the necessity of unifying the approach to calculating the furnace wall temperature with consideration of external conditions. Recommendations are given on designing linings and selecting materials for manufacturing them.