For the problem of the dimensioning of the exchange area of condensation coolers with variable condensate film temperature (employed, for example, in useful heat boilers), and analytic approximation solution is given, which is based on the simplifications by Merkel and a linear approximation of the saturation enthalpy (as function of the temperature). Besides the exchange area, the gas outlet condition as to temperature and humidity can be determined approximately. Comparison calculations in a wide parameter range resulted in deviations in the calculated exchange area between -4% and +1/2%.