A detailed model is proposed for numerical simulation of hydrogen ignition inside box-type passive autocatalytic recombiners (PARs). The model is focused on the reactive channel flow between two catalytic sheets of a recombiner. It includes complex chemistry and multicomponent transport for homogeneous hydrogen combustion and complex surface chemistry for heterogeneous hydrogen recombination. First calculations are dedicated to H2/air mixtures without steam at atmospheric pressure and room temperature. The analysis of the total homogeneous and heterogeneous heat release rates according to the inlet hydrogen molar fraction reveals three possible operation regimes for the recombiners from pure catalytic conversion to pure gaseous combustion. A physical criterion is then proposed for the ignition of H2/air mixtures inside the recombiners. The numerical ignition threshold at 5.4% of hydrogen without steam is in good agreement with experimental data. The criterion is then applied to the ternary diagram including all representative H2/air/H2O mixtures for severe accident conditions in pressurized water reactors. It shows a sharper transition from the catalytic regime to the gaseous one for high hydrogen concentrations. A specific strategy finally allows defining an extended PAR hydrogen ignition limit in the entire ternary diagram, which is well corroborated by the available experimental database.