The objective of this study is to investigate the transient 3D numerical simulations of natural convection of air around a circular finned tube (24.9 mm OD) kept in a small chimney. The annular plain fins are considered in this study. The effects of fin spacing to fin diameter ratio (0.057 mm <= S/Df <= 0.24 mm), chimney height (400-1000 mm) and ambient to surface temperature difference (10 K <= T-s <= 65 K) on the heat transfer and the driving force have been investigated. The results are presented in terms of the temperature contours, velocity vectors, heat transfer and the driving force. It has been found that the heat transfer coefficient increases with an increase in the fin spacing upto an optimum value (S = 8 mm) for all the fin geometries, and beyond S = 8 mm, the heat transfer coefficient decreases. The separation of the thermal boundary layer with a variation in the fin spacing and its effects on the heat transfer and driving force has been shown. For a fixed fin spacing, the heat transfer rate, heat transfer coefficient and the driving force increases with an increase in the fin diameter, however, for D-f > 41 mm, the rate of increase in the heat transfer coefficient reduces. The heat transfer coefficient increases with an increase in the chimney height and it has been found that the effect of chimney height on the heat transfer coefficient is a resultant effect of the air outlet temperature and the driving force generated by the chimney. The base to ambient temperature difference has been varied to observe the temperature sensitivity on the heat transfer coefficient and flow patterns. In the last section, various circular and elliptical tube designs have been investigated, and it is found that, the elliptical tube with minimum ellipticity (b/a = 0.33) and circular tube with smallest diameter (7 mm) provides better heat transfer coefficient than the other circular and elliptical designs. (C) 2015 Elsevier Ltd. All rights reserved.