PURPOSE: To prevent the radiative heat transfer from a bath boiler by a method wherein a shut-off valve is provided in a circulating tube and the shut-off valve is operated by an electromagnet connected in series with a thermostat annexed to the bath boiler.

CONSTITUTION: A bathtub 1 and the bath boiler 2 are connected with the circulating tubes 3 and 19. A short tube 4 fixed to a supporting frame 5 is inserted in the bathtub side opening of the lower circulating tube 3. A lid plate 9 at a shaft rod 7 end is energized in such a manner as to close the bathtub side opening of the tube 3 by providing a spring 8 onto the shaft rod 7 loosely fitted to a supporting plate 6 equipped within the short tube 4. The lid plate 9 is coupled with the lower end of a vertical rod 11 swingingly supported at a fulcrum 10. An iron piece 13 is fixed through a working rod 12 at the upper end of the vertical rod 11 so as to oppose to the electromagnet 14. The electromagnet 14 is connected inseries with the thermostat 15 annexed to the bath boiler 2 to power source in order to open and close the lid plate 9 with the start and stop of heating operation of the bath boiler 2.