A water heater , comprising: a hot water tank that stores a heat medium; a heating heat exchanger (Hx) that heats the heat medium supplied to the hot water tank ; a first pump configured to convey the heat medium to the hot water tank ; a heat medium circuit (C) in which the hot water tank , the first pump , and the heating heat exchanger (Hx) are provided; a lower part temperature sensor (SE8) provided in the hot water tank , the lower part temperature sensor (SE8) being configured to measure a lower part temperature of the heat medium stored in the hot water tank ; an upper part temperature sensor (SE7) provided in the hot water tank and provided higher than the lower part temperature sensor (SE8), the upper part temperature sensor (SE7) being configured to measure an upper part temperature of the heat medium stored in the hot water tank ; and a controller (Cnt1) configured to control the first pump , in a case in which the upper part temperature is lower than a first boiling temperature (Tus1), the controller (Cnt1) being configured to execute a first boiling operation in which the first pump is operated at a first pump rotation frequency, characterized in that in a case in which the upper part temperature is lower than a second boiling temperature (Tus2) that is higher than the first boiling temperature (Tus1) and the lower part temperature is lower than a third boiling temperature (Tds2) that is lower than the first boiling temperature (Tus1), the controller (Cnt1) being configured to execute a second boiling operation in which the first pump is operated at a second pump rotation frequency that is lower than the first pump rotation frequency. a hot water tank that stores a heat medium; a heating heat exchanger (Hx) that heats the heat medium supplied to the hot water tank ; a first pump configured to convey the heat medium to the hot water tank ; a heat medium circuit (C) in which the hot water tank , the first pump , and the heating heat exchanger (Hx) are provided; a lower part temperature sensor (SE8) provided in the hot water tank , the lower part temperature sensor (SE8) being configured to measure a lower part temperature of the heat medium stored in the hot water tank ; an upper part temperature sensor (SE7) provided in the hot water tank and provided higher than the lower part temperature sensor (SE8), the upper part temperature sensor (SE7) being configured to measure an upper part temperature of the heat medium stored in the hot water tank ; and a controller (Cnt1) configured to control the first pump , in a case in which the upper part temperature is lower than a first boiling temperature (Tus1), the controller (Cnt1) being configured to execute a first boiling operation in which the first pump is operated at a first pump rotation frequency, characterized in that in a case in which the upper part temperature is lower than a second boiling temperature (Tus2) that is higher than the first boiling temperature (Tus1) and the lower part temperature is lower than a third boiling temperature (Tds2) that is lower than the first boiling temperature (Tus1), the controller (Cnt1) being configured to execute a second boiling operation in which the first pump is operated at a second pump rotation frequency that is lower than the first pump rotation frequency. <EOS>