Flat residuals
 

I am modeling natural convection in 3dimensional domain. I have a chamber with 3 bars that generate heat. The residuals do not drop by more than 2 orders of magnitude. Also after few iterations the residuals flatten out. I have tried different solution methods like i.e. SIMPLE, PISO, Coupled etc. However after some distortions the residuals flatten out and go to a perfect straight line.

Even reduction in under relaxation factors lead to straight lines after a steep drop. What can be the reason for it?