To provide a system capable of reducing operational availability of an existing boiler.

The system includes: a hot water storage tank 10 connected to a water supply line 2 and a hot water supply line 3 sending out hot water of the existing boiler 1 and capable of receiving water through the water supply line 2 and of supplying hot water through the hot water supply line 3; a heat recovery unit 20 recovering heat of drain hot water discharged from a facility; and a heat pump unit 30 having a heat absorbing means absorbing heat recovered by the heat recovery unit 20 and transferring the heat to a heating medium, a compression means compressing the heating medium, a heating means heating water received by the hot water storage tank 10 by using the compressed heating medium to generate hot water and returning the hot water to the hot water storage tank 10 and an expansion means expanding the compressed heating medium and supplying the expanded heating medium to the heat absorbing means. Hot water generated in the hot water storage tank 10 is supplied to the existing boiler 1 through the water supply line 2 and is heated to a predetermined temperature by the existing boiler 1.