Passive solar air heaters, such as solar chimneys and Trombe Walls, rely on solar-induced buoyancy-driven (natural) convection to produce the flow of air. This thesis reports on an experimental investigation into natural convection in a test rig designed to simulate the operation of lightweight passive solar air-heating collectors. Three test rigs were built of heights 0.5, 1.0 and 2.0 metres, with adjustable channel depths (20 - 150mm) and heat inputs (up to 1000 W/m2). Measurements are made of the air, plate and cover temperatures, the air velocities and heat fluxes, to characterise both steady-state and transient performance. Results are presented as dimensionless correlations of mass flow (as Reynolds number) and efficiency against heat input (as Rayleigh number), channel depth and height. The principle findings of this project were that the mass flow rate increased with increasing height, channel depth and heat input and that this increase in mass flow is governed by 3 independent functions linked individually to height, channel depth and heat input. Thermal efficiency was also shown to be a function of heat input and height (independently), but not of channel depth. The heat transfer co-efficient is a function of heat input and height, while related to channel depth by a weak inverse relationship. Several studies of laminar or turbulent flow in channels have been conducted by others, but in this project flows were found to be mainly transitional. The transient behaviour was also analysed. For these experiments time constants (for heating) ranged typically between 30 and 70 minutes, shorter for the smaller test rigs. Increasing heat input or channel depth also reduced time constants.