The present invention relates to an unpowered small-volume heating boiler and to a backflow-preventing apparatus for the unpowered small-volume heating boiler. The unpowered small-volume heating boiler comprises a water tank, a backflow-preventing apparatus, a heating unit, and a pipe unit through which the water heated by the heating unit and circulated through an area to be heated returns to the water tank. The water is heated by the heating unit and circulated by the pressure generated by the rise in the temperature of the water so as to perform a heating operation, thus simplifying the structure by eliminating the necessity of mechanical elements such as a circulation pump or the like, and achieving economic advantages in manufacturing the boiler. The boiler of the present invention improves portability and mobility and enables easy use, and generates less noise to achieve superior efficiency of use. The boiler of the present invention has superior efficiency in heating water or generating pressures in a heating tank, thus improving water circulation and heating efficiency. The boiler of the present invention condenses steam generated during the heating of the water back into water, thus enabling the boiler to be used in a safe manner over a long period of time without adding water, thereby providing use convenience and safety.
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