A measuring device for a heat exchanger includes a pressure pipe arrangement that is spaced using webs and that has a heating side and an isolation side and that is implemented with at least one heat flow sensor. A heat flow sensor is arranged on the heating side in such a way that at least two temperatures T1 and T2 and the temperature difference thereof can be detected there, and additionally at least one temperature sensor for measuring a temperature T3 is arranged on the isolation side and at a distance to the heat flow sensor.