The invention relates to a method for determining the amount of warm water in a water tank of domestic water heating system, which includes: (a) Pre-determining various situation graphs, each graph describes the variation of the water temperature in said water tank as a function of time, in one specific operational situation of the system; (b) Dividing each of said graphs to plurality of sections, each graph section describes the variation of the water temperature in the water tank as a function of time, during a sectional period of time; (c) Determining for each graph section the percentage of warm water in the tank at the end of said section; (d) Storing in a memory storage said plurality of graph sections, and the corresponding percentage of warm water in the tank at the end of said section, as determined; (e) During the operation of the heating system, sampling periodically the temperature in the tank; (f) For each present sequence of samples, finding in said memory storage the most similar graph section, and further finding in said memory storage the percentage of warm water that corresponds to said graph section, said found percentage define the present percentage of warm water in the tank; and (g) Displaying to the user said present percentage of warm water in the tank, as found.