The invention relates to a heat exchanger (1) for heating fresh water using heat from wastewater in a shower or bathtub. The heat exchanger has a drain pan (3), a heat exchanger unit (2) which is arranged in the drain pan (3), and a distributing element (42) for distributing draining wastewater over the heat exchanger unit (2). The heat exchanger unit (2) has multiple tube portions (24) which follow one another sequentially and are connected to one another by diverting portions (25). Each two horizontally running tube portions (24) which follow one another and which are thus connected by a deflecting portion are arranged one over the other, and wastewater which is dripping or flowing down is sprinkled on or flows over the tube portions one after the other.