This experimental work investigates buoyant flow in a differentially heated vertical channel located inside a water cavity. The flow is found to be highly unsteady, and the key aspect of this study is to consider this unsteady behavior as a succession of states that turn out to be driven by the flow outside the channel. A conditional mean operator with respect to the average wall temperature is used to disentangle the different states through which the flow passes. Most of these states are characterized by a transition from laminar heat transfer in the bottom part of the channel to turbulent heat transfer, with a transition point that moves toward the exit as the average wall temperature increases. For the highest values of the average wall temperature, no transition is observed, and the heat exchange is found to be similar to that along a single vertical plate. For an intermediate range of wall temperature, a transition zone with turbulent heat transfer is observed in the upper part of the channel, and the heat transfer is found to follow the same laws as found for a symmetrically heated channel. For the lowest values of the wall temperature, the beginning of the turbulent zone is observed near the entry. The analysis is extended to several channel widths. The origin of the unsteady behavior is attributed to the flow in the whole cavity, and the conditional mean operator allows characterization of the flow inside the channel independently from the flow in the surroundings.