Abstract: A preheating device for hot water heaters which employs hot gases of combustion from the flue to preheat incoming cold water and to continually preheat water stored in the water tank by natural recirculation. Use of the device provides for increased fuel efficiency because hot combustion gases from the heat source are used for warming of water before venting to the atmosphere, the result being an average increased temperature within the tank so that lesser amounts of fuel are required to reach any desired hot water temperature.