PURPOSE: To permit the utilization of waste heat as well as the easy construction of the bottom heating type bathtub boiler capable of heating water to a predetermined temperature quickly by a method wherein the height of a peripheral flue, provided around the outer peripheral wall surface of the bathtub, is adjusted in accordance with the kind of the bathtub to increase the heat transmitting area of the bathtub.

CONSTITUTION: A thin plate member 1, printed with the diagram of blocks, is extended and laid on a fire grate surface 203. A various kinds of block members, which are correaponding to each other, are arranged to form the block groups 2 of combustion chambers and the bottom heating type bathtub 3 is installed horizontally at a predetermined position on an upper surface 2' so as to interpose a clearance 302 between the inner peripheral rims of the block groups 2, and a water discharging pipe 303 is arranged in a groove 209 for the water discharging pipe, which is formed in the block member 205 of the combustion chamber. A continuous flue space 202, an upper flue communicating space, communicated with said peripheral flue 401, and a smoke discharging port 402 are formed and, thus the flue and smoke discharging block groups 4 are formed by arranging and piling respective members. The thin plate member, showing the arrangement of the boiler blocks, is made of flammable paper and is printed with the diagram of projection in an actual scale.