Provided is a heat source device including a control unit which, in response to a predetermined request for heating, sets a switching valve to a first circulation mode to communicate first and third ports, whereas to cut off communication between second and third ports, and performs a heating operation by actuating a burner and a circulation pump. In response to a predetermined request for hot water supply, the control unit sets the switching valve to a second circulation mode to communicate second and third ports, whereas to cut off communication between first and third ports, and performs a hot water supply operation, by actuating the burner and the circulation pump. A hot water supply stop alarm unit issues a hot water supply stop alarm that urges a user to stop using hot water when the request for heating is continued for a first predetermined time or more.