Temperature lower than possible in closed domain
 

I am working on transient model of natural convection with heat source inside the closed, cylindrical domain and one BC at the wall of cylinder. The BC is contant temperature of 323 K. Initial temperature of air inside this cylinder is equal to 350 K. After a certain time of simulation the temperature inside the lower part of cylinder fall down under 323 even to 293 K. Air is treated as an ideal gas. I have to mention that I also used S2S radiation model.

How is it possible for a closed domain to acquire from calculations a temperature, which lower than BC? Radiation heat transfer is also occuring from higher to lower temp? Could anyone explain it to me?

Best regards,
Czarulla