A burner which is capable of burning a very lean mixture of flammable vapors and air, and which preferably also can burn a mixture of varying richness and B.T.U. content, with the burner desirably including a structure forming an upwardly facing recess and an inlet for introducing the vapor-air mixture into a lower portion of that recess in a relation enabling the mixture, when lean, to burn at least partially in the recess in isolation from external air, and when richer to burn at an upper elevation at which external air can mix with the basic vapor-air mixture.