Backflow condition
 

How does specifying a backflow temperature avoid convergence problems? In the help for fluent I read that this is the case, but I cannot understand the reason behind it. For example I have a fluid domain that is meshed around a hollow box which is getting heated up due to radiation. For the box I have specified the walls as opaque and for the room domain I have given pressure outlet condition with backflow temperature 35 degC. How does this number 35 degC help?