This study is driven by the need to understand requirements to safe blow-down of hydrogen onboard storage tanks through a pressure relief device (PRD) inside a garage-like enclosure with low natural ventilation. Current composite tanks for high pressure hydrogen storage have been shown to rupture in 3.5-6.5 min in fire conditions. As a result a large PRD venting area is currently used to release hydrogen from the tank before its catastrophic failure. However, even if unignited, the release of hydrogen from such PRDs has been shown in our previous studies to result in unacceptable overpressures within the garage capable of causing major damage and possible collapse of the structure. Thus, to prevent collapse of the garage in the case of a malfunction of the PRD and an unignited hydrogen release there is a clear need to increase blow-down time by reducing PRD venting area. Calculations of PRD diameter to safely blow-down storage tanks with inventories of 1, 5 and 13 kg hydrogen are considered here for a range of garage volumes and natural ventilation expressed in air changes per hour (ACH). The phenomenological model is used to examine the pressure dynamics within a garage with low natural ventilation down to the known minimum of 0.03 ACH. Thus, with moderate hydrogen flow rate from the PRD and small vents providing ventilation of the enclosure there will be only outflow from the garage without any air intake from outside. The PRD diameter, which ensures that the pressure in the garage does not exceed a value of 20 kPa (accepted in this study as a safe overpressure for civil structures) was calculated for varying garage volumes and natural ventilation (ACH). The results are presented in the form of simple to use engineering nomograms. The conclusion is drawn that PRDs currently available for hydrogen-powered vehicles should be redesigned along with either a change of requirements for the fire resistance rating or innovative design of the onboard storage system as hydrogen-powered vehicles are intended for garage parking. Further research is needed to develop safety strategies and engineering solutions to tackle the problem of fire resistance of onboard storage tanks and requirements to PRD performance. Regulation, codes and standards in the field should address this issue. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.