A necessary hot water storage amount Wn until hot water storage amount in a hot water storage tank 5 reaches a predetermined set hot water storage amount Wms is calculated from a remaining hot water amount Wm in the hot water storage tank 5 and the set hot water storage amount Wms. 貯湯タンク5の貯湯量が予め定められている設定貯湯量Wmsに達するまでの必要貯湯量Wnを、貯湯タンク5の残り湯量Wmおよび設定貯湯量Wmsから算出する。 - 特許庁