The present invention is a variable flow, multiple pass heat exchanger for use in heating various liquids to include cooking oils. The apparatus is comprised of metallic tubing disposed in a baffled stacked configuration disposed within a combustion chamber or other heating element. The tubular configuration is disposed so as to permit the passage of fluid back and forth across the heating element in an upwardly direction and then to repeat this movement in a downwardly direction through a second tubular configuration. The device is capable of being constructed to permit as many passages as are necessary to achieve the desired result. Flow is controlled by varying the number of the tubular members between sucessive tube layers.