Abstract: Device for sealing retractable soot-blower lances at the point of insertion through the wall of a heat exchanger. A labyrinth box consists of lamellas. The lamellas narrowly surround the soot-blower lances and are secured at the outer edge in a wall box. The wall box is tightly sealed to the wall of the heat exchanger and has an air-barrier connection. The object is to allow one wall box to accommodate several soot-blower lances, even when they have different outside diameters. Several soot-blower lances accordingly extend through the wall box and several lamellas radially surround the soot-blower lances alternately either narrowly or loosely.