The invention relates to a waste heat boiler (1) with variable output, working by natural circulation. The steam/water cylinder (2) functions as a steam (7) and feed water (8) storage tank. All the exhaust gas (52) flows through the convection part (41), whereby the heat transfer surfaces are automatically cleaned due to the high flow velocity. The production of steam (7) is controlled by changing the water level (48) and quantity in the convection part (41). The basis of the control are the different heat transfer characteristics of the liquid and the steam. The steam/water cylinder (2) is connected to the lower end of the convection part (41) by means of the drain pipe (19), wherein is provided valves (20) controlling the water level (48), which may be either continuously adjustable or of the open/closed type. A separate control circuit controls the feed water level in the steam/water cylinder (2). Different feed pipe (6) and baffle (47) arrangements heat the feed water (8) and condense the steam (7).