PURPOSE: To make it possible to conduct a freezing preventing operation even when no water exists in a bathtub by connecting one end of a three-way changeover valve to an outlet pipe or an inlet pipe of a bathtub and causing the three-way valve to conduct a changeover operation by a water level sensor of a bathtub.
CONSTITUTION: One end of a three-way changeover valve 11 is connected to a bathtub outlet pipe 5 immediately in the vicinity of a bathtub 3. A water circulation circuit in the case of water filling the bathtub 3 is formed in a route of bathtub 3→ bathtub outlet pipe 5→ hot water return pipe 6→ pump 7→ heat exchanger 8→ hot water outgoing pipe 10→ three-way changeover valve 11→ bathtub inlet pipe 12→ bathtub 3, and is used as a closed loop circulation circuit upon preventing freezing when no water is present in the bathtub 3. Further, in the case where no water is present in the bathtub 3, a circulation circuit is formed in the route of hot water return pipe 6→ pump 7→ heat exchanger 8→ hot water outgoing pipe 10→ three-way changeover valve 11→ short-circuiting pipe 13→ hot water return pipe 6, and is used as a closed loop circulation circuit at the time of the freezing preventive operation. Accordingly, regardless of the presence or absence of water within the bathtub 3, the water closed loop circulation circuit is formed constantly, and even when no water fills the bathtub 3, the freezing preventive operation becomes possible.