Experiments were performed to determine how the heat transfer from a horizontal finned tube responds to the presence of a sidewall or of a sidewall/floor system. The clearances between the fin tips and the sidewall and between the fin tips and the floor were varied parametrically and, for each configuration, heat transfer results were obtained over a 25-fold range of the tube-to-ambient temperature difference. Measurements were also made for the finned tube situated in free space (i.e. far from any walls), and these results displayed highly enhanced heat transfer (by a factor of 4-8 times) compared to a corresponding unfinned tube. The presence of a sidewall had only a moderate effect on the finned-tube heat transfer, whereas the sidewall/floor system caused greater changes. The extreme reduction in heat transfer due to the latter was about 40%, but reductions in the 10-20% range are more realistic for practical operating conditions. In percentage terms, the presence of the wall(s) had less effect at large temperature differences than at small temperature differences. Measured temperatures at the fin tips, on the sidewall, and on the floor are also presented.