Combustion improvement by renovation of burners in district heating boilers include surpassing allowed nitrogen oxide level and the increased soot discarge. In water heating boilers one additional problem in adjusting of the combustion is a coke formation on cold screen lubes at lower loads. In recent time the fuel oil is heavier and with higher level of asphaltenes and it is more difficult to burn it. Main task of this job is to understand what determines this fuel oil coke formation and how it can be evoided. Calculation of evaporation and burning of liquid fuel drops in the flame ajacent the burner using FLUENT code provided new data on complex mass transfer process in the flame in the vicinity of the burner. Air How created by the burner must be matched and ajusted to fuel streams on the ground of correct design of the recirculation zone. The next important factor is to avoid the zones with lack of oxygen and select a suitable diameter of droplets.