This study is driven by the need to understand requirements to safe blow-down ofhydrogen onboard storage tanks through a pressure relief device (PRD) inside a garage-likeenclosure with low natural ventilation. Current composite tanks for high pressurehydrogen storage have been shown to rupture in 3.5e6.5 min in fire conditions. As a resulta large PRD venting area is currently used to release hydrogen from the tank before itscatastrophic failure. However, even if unignited, the release of hydrogen from such PRDshas been shown in our previous studies to result in unacceptable overpressures within thegarage capable of causing major damage and possible collapse of the structure. Thus, toprevent collapse of the garage in the case of a malfunction of the PRD and an unignitedhydrogen release there is a clear need to increase blow-down time by reducing PRD ventingarea. 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 naturalventilation expressed in air changes per hour (ACH). The phenomenological model is usedto examine the pressure dynamics within a garage with low natural ventilation down tothe known minimum of 0.03 ACH. Thus, with moderate hydrogen flow rate from the PRDand small vents providing ventilation of the enclosure there will be only outflow from thegarage without any air intake from outside. The PRD diameter, which ensures that thepressure in the garage does not exceed a value of 20 kPa (accepted in this study as a safeoverpressure for civil structures) was calculated for varying garage volumes and naturalventilation (ACH). The results are presented in the form of simple to use engineeringnomograms. The conclusion is drawn that PRDs currently available for hydrogen-poweredvehicles should be redesigned along with either a change of requirements for the fireresistance rating or innovative design of the onboard storage system as hydrogen-poweredvehicles are intended for garage parking. Further research is needed to develop safetystrategies and engineering solutions to tackle the problem of fire resistance of onboardstorage tanks and requirements to PRD performance. Regulation, codes and standards inthe field should address this issue.