A water heater system having a running water inlet through which running water flows in and a hot water outlet through which hot water heated in a heat exchanger is discharged, the system comprising: a first connection pipe connecting a gap between the running water inlet and an entry of the heat exchanger; a second connection pipe connecting a gap between an outlet of the heat exchanger and the hot water outlet; a bypass pipe connecting a gap between the first connection pipe and the second connection pipe ; a bypass valve installed to the bypass pipe so as to control the direction of water flowing through the bypass pipe ; and a controller, in a hot water usage mode, controlling the bypass valve so that the water moves from the first connection pipe to the second connection pipe through the bypass pipe , and in an inner circulation mode, controlling the bypass valve so that the water moves from the second connection pipe to the first connection pipe through the bypass pipe , a hot water exchange hole where hot water that is not used from the hot water discharged through the hot water outlet is flowed in; characterized in that the water heater system further comprises: a third connection pipe connected to a gap between a position disposed between the bypass valve and the first connection pipe and the hot water exchange hole ; and a first check valve installed in the third connection pipe , wherein the controller, in the inner circulation mode, controlling the first check valve to be turned off and in an outer circulation mode or a complex circulation mode, controlling the first check valve to be turned on. a first connection pipe connecting a gap between the running water inlet and an entry of the heat exchanger; a second connection pipe connecting a gap between an outlet of the heat exchanger and the hot water outlet; a bypass pipe connecting a gap between the first connection pipe and the second connection pipe ; a bypass valve installed to the bypass pipe so as to control the direction of water flowing through the bypass pipe ; and a controller, in a hot water usage mode, controlling the bypass valve so that the water moves from the first connection pipe to the second connection pipe through the bypass pipe , and in an inner circulation mode, controlling the bypass valve so that the water moves from the second connection pipe to the first connection pipe through the bypass pipe , a hot water exchange hole where hot water that is not used from the hot water discharged through the hot water outlet is flowed in; characterized in that the water heater system further comprises: a third connection pipe connected to a gap between a position disposed between the bypass valve and the first connection pipe and the hot water exchange hole ; and a first check valve installed in the third connection pipe , a third connection pipe connected to a gap between a position disposed between the bypass valve and the first connection pipe and the hot water exchange hole ; and a first check valve installed in the third connection pipe , wherein the controller, in the inner circulation mode, controlling the first check valve to be turned off and in an outer circulation mode or a complex circulation mode, controlling the first check valve to be turned on. <EOS>