Partially filled interconnected vessels are investigated using hydrogen-air mixtures in the lean flammable range. Homogeneous hydrogen-air mixtures (10-22% by volume) were prepared in a 100 L cylindrical vessel which was connected to a larger (400 L) vessel (containing air) through a 1m connecting pipe. The mixture was ignited at the flange, directly opposite the connecting pipe. For concentrations between 12 and 16% pressure piling was displayed in the 400 L vessel. For concentrations between 18 and 22% an additional detonation event in the primary vessel was observed, demonstrating the potentially severe consequences of hydrogen leakage and ignition in an enclosed interconnected geometry. The cause of this deflagration to detonation transition (DDT) is unclear and is divorced from the flame propagation in the duct (attributed as the origin of DDT in similar systems by other workers). In the present research, the DDT appears similar to the "engine knock" effect or hot spot initiation of autoignition/detonation.