A candle oven based on the most efficient wood fired home heaters - the kajelofen. After 45 minutes with a couple of tea lights in the bottom, this gets too hot to touch in the upper compartment, and the whole piece radiates heat for hours afterward.
DIY HOME Candle Powered Heater - I've been looking a various ways to make these and this guy has by far the best setup. It isn't so much a how-to video. He is just showing what he used to construct his version of the terra cotta pot candle heater.
Warm a Room with Only a Candle and a Few Terra Cotta Pots.The terra cotta pots concentrate the heat using convection. While you won’t heat a whole house with it, it can definitely warm your hands and make a cold night much more bearable .