For coupled wall, the general usage scenario of UDF macro DEFINE_HEAT_FLUX is to impose the heat flux from fluid side and then let Fluent do the thermal coupling between fluid and solid zones. So if you are affecting the macro DEFINE_HEAT_FLUX on the wall bc attached to the fluid side, you should not attach the same macro to the corresponding bc on the solid side. There is also an important note in Fluent documentation (see section 2.3.17. DEFINE_HEAT_FLUX of Fluent Customization Manual) on this point: *** Important: This function allows you to modify the heat flux at walls adjacent to a solid. Note, however, that for solids since only heat conduction is occurring, any extra heat flux that you add in a heat flux UDF can have a detrimental effect on the solution of the energy equation. These effects will likely show up in conjugate heat transfer problems. To avoid this, you will need to make sure that your heat flux UDF excludes the walls adjacent to solids, or includes only the necessary walls adjacent to fluid zones. *** Keywords: DEFINE_HEAT_FLUX, coupled wall, wall shadow