The use of stationary hydrogen and fuel cell systems is expected to increase rapidly in the future. In order to facilitate the safe introduction of this new technology, the HyPer project, funded by the EC, developed a public harmonized installation permitting guidance (IPG) document for the installation of small stationary hydrogen and fuel cell systems for use in various environments. The work was strongly supported with experiments and related CFD simulations. The present contribution focuses on the safety assessment of a facility, inside which a small hydrogen fuel cell system (4.8kWe) is installed and operated. Dispersion experiments were designed and performed by partner UNIPI in collaboration to partner KI. Partner NCSRD assisted in the design by performing related pre-test calculations. The scenarios considered cover releases occurring inside the fuel cell, related to the low pressure hydrogen downstream of the pressure regulator, which controls the hydrogen flow to the fuel cell system. Hydrogen is expected to leak out of the fuel cell into the facility and from there outside in the open through the facility ventilation system. The initial leakage diameter was chosen based on the ATEX limits for classification of the facility as Zone 2. The release flow rate was calculated assuming 5 bars pressure. Two higher release flow rates (larger leak diameters) were also considered to address more dangerous cases. Several natural ventilation configurations were examined. The performed tests were simulated by NCSRD using the ADREA-HF code. The performed analysis took into account the full interior of the fuel cell, in order to investigate for any potential accumulation effects. The present contribution focuses on the performed CFD calculations. Comparisons between predicted and experimental hydrogen concentration at 4 sensor locations inside the facility are reported. Based on the performed calculations and experiments, an overall assessment of the ventilation efficiency of the facility is made.