We have performed numerical calculations to analyze the low‐frequency properties of a single‐junction thermal converter using a modified low‐frequency model. A feature of the modified low‐frequency model is the incorporation of the heat capacity of the thermocouple and ceramic bead, which had been previously neglected. Since thermocouple and ceramic bead can increase the thermal inertia of the heater, they can modify the thermal time constant, which plays a crucial role at low frequencies. The calculated data using modified low‐frequency model for the AC–DC transfer differences correspond better to the experimental data. The difference of the AC–DC transfer difference between calculated and measured data is less than 10 µV/V down to 10 Hz. IEEJ Trans 2012. doi: 10.1002/tee.21739