Abstract: The invention relates to a device for passive heat transfer between a heat source and a condenser (6) by means of a vaporizable liquid and a cycle with two phases controlled by a float valve (2, 4, 11). This device comprises a boiler (1) heated by said source and a liquid tank (9), the boiler furnishing, during a first cycle phase, vapor to the condenser which then supplies the tank with liquid, and receiving, during the second cycle phase, liquid from said tank. According to the invention, a vapor separator (3) is interposed between the boiler and the condenser, and the float valve (2, 4, 11) is disposed inside this separator and makes communicate, during the first cycle phase, the vapor space of the separator with the vapor space of the boiler (1), and, during the second cycle phase, the vapor space of the separator (3) with the vapor space of the tank (9), while cutting off these communications during the respective inverse cycle phase.