primary side at the upstream connector 6 is 65 u00b0 c and the inflow temperature of the consumable water at the inflow connector 11 is 10 u00b0 c the temperature increases the longer the supplied consumable water moves into the heat exchanging stretch with the application of the second temperature 24 in the middle that is at an effective heat exchange surface of 0 5 a temperature of about 34 u00b0 c is measured at a lower loading of 15 the temperature of the heat exchanger at the middle falls to 28 u00b0 c as is shown by the characteristic line b the characteristic lines c and d show respectively the temperature course at normal loading and at 15 loading when the inflow temperature of the primary side is at 100 u00b0 c the characteristic lines of the heat exchanger change greatly with the loading and the inflow temperature and the temperature change is greatest in the middle of the heat exchanger so that the temperature sensor 24 is advantageously applied here finally it is also possible from the temperature course with the help of the temperature sensors that is at least with the help of the second temperature sensor 18 or with the help of the third temperature sensor 19 to conclude that a u201c fouling u201d has occurred that is that a slow clogging of the heat exchanger has taken place as is to be recognized from fig1 the temperature sensors 16.