a relatively small water pump - dc motor combination , powered by batteries , provides sufficient pump power to create the flow between the hot and cold water lines necessary to maintain a substantially immediate hot water response when the hot water tap is opened . the pump is controlled by a thermal sensor signaling the pump to commence operation when the temperature in the hot water line upstream of the hot water tap is less than a predetermined value . preferably , each of the batteries is rechargeable , most preferably utilizing a wireless system , where the power source is a distance away from the pump battery , to provide additional protection against electrical short circuits in the presence of water . commonly available batteries of between 10 and 24 volts can be used .