Hot surface ignition is relevant in the context of industrial safety. In the present work, two-dimensional simulations with detailed chemistry, and study of the reaction pathways of the buoyancy-driven flow and ignition of a stoichiometric hydrogen-air mixture by a rapidly heated surface (glowplug) are reported. Experimentally, in hydrocarbon-air, ignition is observed to occur regularly at the top of the glowplug; numerical results for hydrogen-air reproduce this trend, and shed light on this behavior. Flow separa- tion plays a crucial role in creating zones where convective losses are minimized and heat diffusion is maximized, resulting in the critical conditions for ignition to take place.