Abstract: A room heating hot water boiler is disclosed in which a plurality of longitudinal rectangular hexahedral water tank groups and a plurality of lateral hexahedral water tanks are disposed in four steps above a combustion chamber in such a manner as to communicate with each other, and guide projections are formed so that an exhaustion gas discharge conduit should be provided in a zig-zag form. Consequently, the heat transfer area is increased, and convection occurs through small and large paths, so that the water of the boiler can be speedily heated, thereby improving the heat efficiency of the boiler. A lower water tank 3 and a recovering tank 3A are formed around a combustion chamber 2 of the boiler body 1. A motor pump 4 is connected between the two water tanks 3 and 3A, and a plurality of longitudinal water tank groups 3a and 3c and a plurality of lateral water tanks 3b and 3d are alternately stacked in four steps.