PURPOSE: To prevent the water level switch in a hot water storage tank from malfunctioning by a structure wherein a guide plate is provided within the hot water storage tank in order to prevent the return water from hot water heating from causing vortex flow within the hot water storage tank.

CONSTITUTION: Water discharged from the hot water storage tank 2 by means of a pump 3 is at first heated at an heat exchanger and, after that, used at a radiator and a bath heater and sent through a heating return circuit back to the hot water storage tank 2. A by-pass pipe represents the by-pass circuit of the heating return circuit. In this case, because of the guide plate 6 provided within the hot water storage tank 2, return water is smoothly guided to the suction port of the pump 3, resulting in generating no vortex flow and consequently in causing no malfunction of the water level switch 1.