Experiments were performed in a 25m3 vented combustion test facility that hosted internally a 0,64m3 vented box, called RED-CVE, which vent was left open. Test facility vent area was fixed, set to a value of 1,12m2, while three different areas were tested for the RED-CVE vent. Hydrogen concentration was homogenised inside the two environments by use of two different fans. The study included hydrogen concentration from 8,5%2to 12,5%2 while three different ignition locations were used, (1) far vent ignition, (2) ignition in the middle of the rear wall of the RED-CVE box, and (3) near vent ignition. Peak overpressures generated inside the test facility as well as inside the smaller box were investigated. Experiments indicate that peak overpressures inside the CVE test facility with near vent ignition were negligible compared with those with far vent ignition and ignition inside the RED-CVE box. In experiment with far vent ignition the pressure at first increased with hydrogen concentration, reached a peak value at 11%2ol. concentration, than decreased showing a non-monotonic behaviour. Overpressure produced inside the RED-CVE resulted higher when ignition location was external to the box, with the flame entering in the RED-CVE through the vent, than when the mixture was ignited directly inside. Key words: Hydrogen, Vent, Obstacles, External ignition