A total of 12 ventilation experiments with various combinations of hydrogen release rates andventilation speeds were performed in order to study how ventilation speed and release rate effect thehydrogen concentration in a closed system. The experiential facility was constructed out of steel platesand beams in the shape of a rectangular enclosure. The volume of the test facility was about 60m3.The front face of the enclosure was covered by a plastic film in order to allow visible and infraredcameras to capture images of the flame. The inlet and outlet vents were located on the lower front faceand the upper backside panel, respectively. Hydrogen gas was released toward the ceiling from thecenter of the floor. The hydrogen gas was released at constant rate in each test. The hydrogen releaserate ranged from 0.002 m3/s to 0.02 m3/s. Ventilation speeds were 0.1, 0.2 and 0.4 m3/s respectively.Ignition was attempted at the end of the hydrogen release by using multiple continuous spark ignitionmodules on the ceiling and next to the release point. Time evolution of hydrogen concentration wasmeasured using evacuated sample bottles. Overpressure and impulse inside and outside the facilitywere also measured. The mixture was ignited by a spark ignition module mounted on the ceiling ineight of eleven tests. In the other three tests, the mixture was ignited by spark ignition modulesmounted next to the nozzle. Overpressures generated by the hydrogen deflagration in most of thesetests were low and represented a small risk to people or property. The primary risk associated with thehydrogen deflagrations studied in these tests was from the fire. The maximum concentration isproportional to the ratio of the hydrogen release rate to the ventilation speed within the range ofparameters tested. Therefore, a required ventilation speed can be estimated from the assumedhydrogen leak rate within the experimental conditions described in this paper.