System for detecting a blockage in a heat exchanger in a water heater, wherein the system comprises the water heater, which comprises a first pipe, which is able to be connected hydraulically to a central heating circuit, and a second pipe, which is able to be connected hydraulically to a domestic water pipe, a heating cell, which is connected to the mentioned first pipe to heat the liquid in the first pipe, a first volume, which is connected hydraulically to the first pipe, and a second volume, which is connected hydraulically to the mentioned second pipe, at least one heat transfer means, which is provided between the mentioned first volume and the mentioned second volume, wherein the at least one heat transfer means, the first volume and the second volume form a heat exchanger to transfer heat between the first pipe and the second pipe, wherein the system comprises a temperature sensor to measure the temperature of the liquid in the second pipe, a meter to record the activation and deactivation states of the mentioned heating cell, a first flow measuring device to measure the volume flow of the liquid in the first pipe, a second flow measuring device to measure the volume flow of water in the second pipe, a control unit, which is connected to the temperature sensor, the first flow measuring device, the second flow measuring device and the meter and therefore receives the temperature, flow and state meter measurements, characterized in that the control unit is configured in such a way that it assigns the received temperature measurements, flow measurements and the information about activation and deactivation states to the reception times thereof and stores them in a memory unit, and - in that the stored temperature measurements, flow measurements and information about the activation states are monitored, - in that a signal about the blockage in the second volume of the heat exchanger is generated if an upward trend in the temperature measurements from the temperature sensor is detected in a first time interval, if it is determined that the flow measurements obtained from the first flow measuring device are within a predetermined range of values in the mentioned first time interval, if a downward trend in the flow measurements obtained from the second flow measuring device is determined in the first time interval, and if an upward trend in the frequency of information from the meter about the activation and deactivation states is determined in the first time interval. - in that the stored temperature measurements, flow measurements and information about the activation states are monitored, - in that a signal about the blockage in the second volume of the heat exchanger is generated if an upward trend in the temperature measurements from the temperature sensor is detected in a first time interval, if it is determined that the flow measurements obtained from the first flow measuring device are within a predetermined range of values in the mentioned first time interval, if a downward trend in the flow measurements obtained from the second flow measuring device is determined in the first time interval, and if an upward trend in the frequency of information from the meter about the activation and deactivation states is determined in the first time interval.