A method for operating a heat exchanger, through which a heat transfer medium flows on a primary side, entering the heat exchanger with a first temperature and exiting the heat exchanger with a second temperature. The heat transfer medium emits on a secondary side a heat flow to a secondary medium flowing through the heat exchanger in the case of heating or, in the case of cooling, absorbs a heat flow from the secondary medium which enters the heat exchanger with a third temperature and exits the heat exchanger again with a fourth temperature. The heat exchanger is capable of transferring a maximum heat flow. At least three of the four temperatures are measured and the respective saturation level of the heat exchanger is determined from these measured temperatures and is used for controlling the operation of the heat exchanger.