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During their initial development, passive autocatalytic hydrogen recombiners (PARs) were presumed to operate in a flameless mode. However, a series of independent experiments conducted in the 1990s observed hydrogen-air gas mixtures igniting as a result of PAR operation. This ignition was due to overheating of the catalyst, leading to the thermal ignition of hydrogen-air mixtures (termed “internal ignition”). Additionally, individual particles may become detached from the catalyst substrate and swept up by the gas stream to subsequently ignite the gas mixture outside the recombiner housing, a phenomenon known as “external ignition.” This article delves into the experimental findings concerning two mechanisms of hydrogen-air mixture ignition. Direct evidence for recombiner-induced external ignition was captured using the Schlieren method. It was confirmed that the concentration limits for external ignition differ from those for internal ignition. In order to ensure nuclear power plant safety, the development of a testing methodology for the technology used in manufacturing catalysts is essential.