A numerical study has been performed comparing the hazards, in particular overpressures, arising from the sustained unignited and ignited release from an onboard hydrogen storage tank at 700 bar through a 3.34 mm diameter orifice, representing a thermally activated pressure relief device (TPRD) in a small garage with a single vent equivalent in area to small window. It has been demonstrated how the overpressure predicted in the case of an unignited release using both CFD and an analytical model is in the region of 0.55 kPa and thus unlikely to cause structural damage. However, the overpressure predicted for the ignited release is two orders of magnitude greater, reaching over 55 kPA in less than 1 s and thus potentially causing destruction of the structure. It has been shown that whilst the overpressures resulting from the unignited release are unlikely to cause harm, the garage is engulfed by a flammable atmosphere in less than 1 s and the oxygen is depleted to levels dangerous to people within this time. In the case of the ignited release, whilst the resultant overpressures are the primary safety concern, it has been shown how the thermal effects resulting from the release extend almost 9 m from the jet in 1.5 s. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.