Modelica mass and energy balance

edit

Is there a good rule-of-thumb for selecting the appropriate energy and mass balances for thermo-fluid components in Modelica (i.e. free initial / fixed initial / steady state initial / steady state)? The MBL User Guide says “Fixed Initial” should be used with caution, but most examples seem to use that. I am calibrating a model with fluctuating temperature and flow rate conditions, and I am not sure how to apply these settings. Any guidelines or recommended reading would be greatly appreciated.