Abstract This paper discusses the close interaction between local fluid dynamics and natural convection heat transfer from a pair of isothermally heated horizontal cylinders submerged in water. The presence of a second heated cylinder induces heat transfer enhancements of up to 10%, and strong fluctuations in local heat transfer rate. Therefore specific attention is focused on how the local heat transfer characteristics of the upper cylinder are affected by buoyancy induced fluid flow from the lower cylinder. The paper investigates a range of Rayleigh number between 1.8 × 10 6 and 5.5 × 10 6, and a vertical cylinder spacing between 2 D and 4 D. Simultaneous local heat flux measurements and flow velocity measurements using particle image velocimetry reveal oscillatory behaviour of the thermal plume, depending on operating conditions. A joint temporal analysis of the data has provided new insights into the governing mechanisms, which enables further optimisation of the heat transfer performance.