Because of the temperature dependence of the carrier liquid density, the mass of carrier which is contained in a thermal field-flow fractionation channel depends on the cold wall temperature and on the temperature difference across the channel thickness. It is observed that the void time of the solvent peak decreases when increasing the average temperature in the channel. The void time is found to be directly proportional to the average carrier density in the channel. The determination of the void time from the knowledge of the channel geometrical volume and the measurement of the volumetric flow-rate leads to significant errors if the thermal expansion of the carrier between the temperature of the measurement and the average channel temperature is not taken into account. Recommendations are given for proper void time determinations in thermal FFF.