The first step in the development of a fuzzy control algorithm is defining membership functions for all the variables involved, i.e. for all inputs and outputs of the process regulator. In this application, there is at least one input variable,which is temperature error TE (the difference between set and measured temperature). The two output variables are heating power H (resulting from regulated electric current through the electric heater, H for heater) and cooling power V (resulting from the inrush of cold water, i.e. from the valve setting, V for valve).