The effect of interplate spacing on natural convection in an open-ended vertical channel bounded by an isothermal and an unheated wall was studied both experimentally and computationally. The investigation encompassed the full range of operating conditions of the channel, i.e. from the limit of the fully-developed channel flow to the limit of the single vertical plate. Overall, a 50-fold variation in the spacing between the channel walls was investigated, and the vertical plate was employed in order to achieve the limit of infinite spacing. The experiments were performed in water (Pr ≊ 5) with the aforementioned parametric variation of the interplate spacing and for an order of magnitude range of the wall-to-ambient temperature difference. The numerical solutions were carried out for the experimentally investigated operating conditions and took account of both natural convection in the channel and conduction in the wall. It was found that the flat plate heat transfer does not form an upper bound for the channel heat transfer. The channel heat transfer is particularly sensitive to changes in interplate spacing for narrow channels and at small temperature differences. The results for all operating conditions were brought tightly together in terms of the groups Nus and (S/H)Ras, and a highly accurate correlation encompassing eight decades of (S/H)Ras, was developed. Excellent agreement between the experimental results and the computational predictions was obtained.