Strip surface state before zinc bath immersion is linked to temperature homogeneity and gas atmosphere composition in each furnace section. Many different gas metal reactions occur in the furnace that may improve the strip heating rate or introduce surface defects, depending on their localization. Gas flow models of the preheating-heating sections with high temperature variations have been developed. They point out temperature gradients and the competition between natural convection and forced convection. A model for gas exchanges between all furnace sections has been built and validated by gas tracing trials on industrial lines. The effect of line process parameters such as line speed, strip format and gas injection condition in all furnaces sections is reported.