During retrofits taken on power unit “Nikola Tesla” A6 in 2011 for the purpose of increasing its overall production capacity certain modifications on milling plants have been applied. Enlargement of power steam boiler production capacities included increase in milling plant output with existing number of mills in operation. Changes made on arrangement of pulverized coal burners, geometry of mill gaseous mixture distribution elements inside mill gaseous mixture duct as well as operational rotational speed of mill impeller led to increase of milling capacity to the certain extent. Consequently, applied modifications resulted in higher primary air-flow required for the purpose of regulating mill gaseous mixture temperature, thus, reducing secondary airflow in burners below recommended level which affected combustion process within boiler furnace. In this paper results of thermal calculations of power steam boiler and aerodynamic calculations of milling plant for the present operating conditions have been presented. Calculations results have been validated by use of measurements taken on site in present operating conditions. Impact on overall boiler operation made by modifications taken on milling plants has been discussed in details. In text new measures, which would reduce occurred negative effects, have been proposed. Positive effects of proposed measures, as new operating point of milling plant, are confirmed by the results of additionally performed thermal and aerodynamic calculations given in this paper.