A method and apparatus for controlling the operation of a water lance for water cleaning a furnace wall comprises a sensor or calculation equipment to derive the furnace wall emissivity and equipment that compares the emissivity to a programmed low setpoint that represents an unclean furnace wall and poor furnace operation. Water lance operation is initiated when the low setpoint is reached. The speed at which the water lance operates is also adjusted according to peak emissivity levels which are measured or derived from the furnace walls after each cleaning.