Hydrogen-based decentralized energy systems (H2-DES) are gaining prominence as potential solutions for local decarbonization of heat and power supply. This paper presents a life cycle assessment (LCA) of a H2-DES using an internal combustion engine for combined heat and power (ICE-CHP), grounded on a techno-economic analysis (TEA) model implemented in OpenModelica. The study addresses the relative underexploration of H2-ICE-CHP technologies in the LCA literature, offering a comprehensive analysis based on real hourly demand data of buildings in Offenburg (Germany), a municipality involved in the Franco-German Interreg CO2InnO project. Scenarios families were developed with varying configurations and dimensioning (PV, wind, battery, H2 storage). The results of the TEA model were interfaced with the Brightway2 framework for LCA through custom Python-based routines and manual implementation. Monte Carlo simulations and global sensitivity analysis were conducted in the Activity-Browser software to evaluate uncertainty and parameter influence. Results show that impacts are strongly dependent of the scenario family and selected system boundaries. The H2 sub-system components showed modest impacts to climate change but not in other environmental categories, while PV significantly affected environmental assessment overall, depending on excess electricity consideration. These findings highlight the necessity of careful system design and boundary comparison in policy and environmental reporting. This work also contributes methodologically towards the development of an open-source TEA-LCA workflow. While still requiring manual data adaptations, it forms the foundation for future automation efforts. The approach offers actionable insights for local policymakers and researchers aiming for transparent, reproducible, and multi-criteria sustainability assessments of H2-DES.