Abstract: A controlling method for a hot water system and a thermal storage tank for a hot water system are provided in the present invention. The method includes the steps of: (a) turning on the pump to circulate the first fluid and to provide the heat collector with the first fluid; (b) turning off the pump after the pump is on for a first predetermined time to hold the first fluid in the heat collector; (c) turning on the pump after the pump is off for a second predetermined time to circulate the first fluid again; and (d) returning to the step (b). The thermal storage tank holding a first fluid and comprising: a first inlet inputting the first fluid; a first outlet outputting the first fluid; a first inner tank holding a second fluid, exchanging a heat between the first fluid and the second fluid and having a second inlet and a second outlet; and a second inner tank having a first heater heating the first fluid, a third inlet connected to the first outlet and a third outlet outputting the heated first fluid.