We compare the point source and solid flame models for assessing the thermal flux levels from hydrogen vent flares. Substantial disagreement exists between the solid flame approach and the point source model close to the base of the flare stack. In this region, the solid flame model is more reliable and may result in higher estimates of the maximum thermal flux from the flare stack. We use the solid flame model to determine restrictions on vent heights and the location of buildings. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.