REDUCTION AMOUNT CALCULATING DEVICE, AND SENSOR FAILURE DETECTION METHOD THEREFOR

PROBLEM TO BE SOLVED: To provide a reduction amount calculating device capable of detecting failure of a flow rate sensor, and a sensor failure detection method therefor.SOLUTION: A reduction amount calculating device 5 is used for a solar heat water heater system 1 having a solar heat water heater 2 for heating supplied water to produce pre-heated hot water, and a water heater 4 for heating pre-heated hot water supplied from the solar heat water heater 2, and calculates a reduction amount that has been allowed to reduce when the pre-heated water is heated in the water heater 4 by heating of the solar heat water heater 2. The reduction amount calculation device 5 calculate the reduction amount based on a first temperature sensor 51, a second temperature sensor 52, and a flow rate sensor 53, and signal values from these sensors 51 to 53. Further, if the second temperature sensor 52 detects a temperature rise, the reduction amount calculation device 5 detects failure of the flow rate sensor 53 based on whether a flow rate value of the flow rate sensor 53 has increased.