Abstract: A quadrangular type multi-tube once-through boiler having an arrangement in which a feed path combuation air, combuation chamber, and an exhaust gas channel passing through a flue are alined in substantially the same plane, and provided with a burner duct (5) and a blower (4) each installed in any desired regions in lateral wall portions (s.sub.1, s.sub.2, s.sub.3 and s.sub.4) defining the width of the boiler body (1). The boiler body (1) comprises an assembly (A) of a plurality of substantially vertically disposed water tubes (10), so that combuation gas flows crosswise of the water tubes (10). A burner (3) is disposed in close adjacency to the first row (1.sub.1) of water tubes the water tube assembly (A), whereby even if fuel from the burner is ignited in a space between the first row (1.sub.1) of water tubes and the burner (3), actual combuation does not proceed to completion therebetween but the most of the unburnt gas burns completely while it flows through water tube clearances.

Abstract: A quadrangular type multi-tube once-through boiler having an arrangement in which a feed path combuation air, combuation chamber, and an exhaust gas channel passing through a flue are aligned in substantially the same plane, and provided with a burner duct (5) and a blower (4) each installed in any desired regions in lateral wall portions (S.sub.1, S.sub.2, S.sub.3 and S.sub.4) defining the width of the boiler body (1). The boiler body (1) comprises an assembly (A) of a plurality of substantially vertically disposed water tubes (10), so that combuation gas flows crosswise of the water tubes (10). A burner (3) is disposed in close adjacency to the first row (l.sub.1 of water tubes the water tube assembly (A), whereby even if fuel from the burner is ignited in a space between the first row (l.sub.1) of water tubes and the burner (3), actual combuation does not proceed to completion therebetween but the most of the unburnt gas burns completely while it flows through water tube clearances.