Abstract: A vapor generator is made up of a tube wall with conducting tubes which are gas-tightly welded together followed by a sheet metal flue gas duct leading to a stack. The tube wall has a temperature zone in which the temperature difference between the medium in the tubes and the flue gas is at its minimum under any operational conditions. In addition to the conducting tubes, the tube wall has a portion with non-conducting tubes which are gas-tightly welded together and through which the medium is non conducted. The non-conducting tubes are aligned with respective tubes of the conducting portion of the tube wall with the interposition of tube transition pieces or T-shape pieces. The T-shape transition pieces include a connection to the conducting tube with a right angle tube leg for the offtake of the vapor and an aligned leg which is welded to the non-conducting tubes of the non-conducting tube section.