Problem with radiation heat transfer
 

Hi, I'm simulating a heating process of a slab within a furnace. I imposed the fixed values of temperature for the furnace gas and its walls . I also imposed velocity zero to the fluid.
The model just calculates the radiation heat transfer between the slab and the furnace, and resolves the equation of conduction of heat within the slab.

What I see in the results its something that is impossible!!!
Fluent is telling me that the total surface heat flux is higher than the radiation heat flux on the slab surfaces.
How could that be, if there is no convection, just radiation.

Anyone could provide some theory that would explain these results?

Thank you