Heat-sensor electrodes without electrical insulation between the working electrode and the thermistor were made. A lumped-heat-capacity analysis was applied to the electrodes and two kinds of energy balance equation were derived. The correctness of measurements in the case of the presence of an electrical contact between measuring and polarization circuits was confirmed experimentally. Problems such as the legitimacy of the lumped-capacity analysis, the overpotential and the relationship between the temperature change of the working electrode and that of the thermistor are discussed. This paper gives the foundations for the determination of the electrochemical Peltier heat in the following paper.