1) If in a thermal simulation, the case geometry has many shell conduction zones, particularly if they are on the walls not connect to each other, there is a chance that the solution will show some heat energy imbalance even if all the residuals and monitors look converged 2) the reason for this observation is, the side walls for a shell conduction zone, if not connected to any adjacent shell layers from another walls, are by default treated as adiabatic 3) hence, if there are many such walls in your case, the heat imbalance at these side of shell layers amount to the overall heat imbalance 4) this can be particularly observed in cases having a lot of baffles modelled using multi-layer shell conduction model for ex., engine head fins, Vehicle Underhood Thermal analysis with multiple baffle frames modelled as shell layers, etc.