Abstract: For heating a body of liquid, such as water within a domestic water tank, there is provided an elongated hollow tube extending at an angle with respect to the vertical from the bottom to the upper region or level of the water. The tube is closed at its bottom and open at its top. A series of openings are formed along the lowermost generatrix of the tube. Heating the water inside the lower region of the tube will cause a thermal circulation whereby warmed water will rise along the uppermost generatrix of the tube, and cold water will enter through the openings, flow first downwards and, after getting warmer, rise with the warm water upwards flow. The upper region of the body of water will thus become more intensively heated than the rest of the water.