The present invention relates to a hot water heat transfer pipe that exchanges heat between its interior and exterior. A plurality of projections, each having a height in the range of 0.8-2.0 mm or 0.1-0.25 times the inner diameter, is provided in at least one part of an inner surface of a portion of the heat transfer pipe positioned in a section where the Reynolds number of the fluid flowing in the interior is less than 7,000 to improve the heat transfer performance in the low Reynolds number zone, and minimize pressure loss inside the pipe.