a helical coil of tubing is disposed in a cylindrical tank in spaced relation thereto so as to act as a baffle for the products of combustion introduced thereinto by creating a tortuous passageway in said tank . the helical coil has its end portions arranged in spaced parallel sections defining supporting skids extending longitudinally of the helical coil and thereby supporting the same in said tank . the tank is positioned in a secondary larger cylindrical tank and the fluid to be heated , such as oil , is circulated through the helical coil , the skid - like communicating portions thereof forming the supports and in the area between the tanks to obtain a highly efficient heat transfer .