Sectional boiler

A sectional boiler with a combustion chamber and with water-bearing boiler sections (1, 2) delimiting a waste gas guide, has boiler sections (1) which delimit both a horizontal part (3) passing through them of the waste gas guide and the combustion chamber (4). In the direction of flow of the waste gases, there are arranged in a selectable quantity after these boiler sections (1) further boiler sections (2) which only delimit this part (3) of the waste gas guide and themselves only serve for waste gas guidance and cooling. These boiler sections (2) can within their outer parts, which form the boiler walls, be composed of narrow, vertically running inner parts (8) which extend from the horizontal part (3) of the waste gas guide inside a vertical part (9) of the waste gas guide to a waste gas outlet (10) arranged in the bottom region of the boiler.