This work deals with transient natural convection in a vertical channel with wall heating, the channel being located inside a cavity. This experimental work is focused on the first hours after the onset of the heating. The aim of this study is to investigate the link between the heat transfer in the channel with respect to the flow outside the channel. A thermal model of the fluid above the channel is performed and the result fits well with the experimental data. Moreover, the model leads to an expression for the ascending mass flow rate in the channel by using temperature measurements which is an important result because direct measurements remain a tricky problem. This study confirms the approach developed by the same team in a previous work