The invention relates to a steam generator whose continuously heating evaporator surface is arranged in a hot gas channel for a hot gas passage in a substantially horizontal direction and which comprises a plurality of steam generating pipes passed through by flowing medium, wherein a plurality of output collectors which are mounted downstream of certain steam generating pipes, on the side of flowing medium, are oriented in the longitudinal direction thereof substantially parallel to the hot gas direction. The invention improves the steam generator such that it is possible to attain a high operational flexibility, a particularly reduced start- and load-alternation time, including starting operation or light load phases and to maintain a low-cost production. Each output collector comprises an integrated water separating element by means of which said collector is connected, on the side of flowing medium, to a plurality of downstream arranged overheating pipes of an overheating surface.