A heat exchanger for heating fresh water using heat from wastewater in a shower or bathtub. The heat exchanger has a drain pan, a heat exchanger unit that is arranged in the drain pan, and a distributing element for distributing draining wastewater over the heat exchanger unit. The heat exchanger unit has multiple tube portions that follow one another sequentially and are connected to one another by diverting portions. Each two horizontally running tube portions that follow one another and that 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.