A double-tube heat exchanger includes an outer tube and an inner tube forming a first annular gap. The outer tube is provided with an inlet connection and an outlet connection for inletting and outletting a first fluid flowing in the first annular gap. The inner tube includes a first inlet connection and a second outlet connection for inletting and outletting a second fluid flowing in the inner tube for an indirect heat exchange with the first fluid. One of the tube sections is integrally formed with an assembly wall which joints a first end of the outer tube to the inner tube, to seal the first annular gap at the first end of the outer tube. A second annular gap is exposed to the air and is in fluid communication neither with the first annular gap nor with the inner tube, and is partially surrounded by the first annular gap.