Vent devices for gas and dust explosion are often ducted to safety location by means of relief pipes for the discharge of hot combustion products or blast wave. Relief pipes are specifically required when the jet flowing violently from the vent area has to be avoided, e.g. within buildings. On the other hand, the presence of a duct is likely to increase the severity of explosion if compared to simple vented vessels. Despite these considerations, the combustion phenomenon is not fully understood so far, and few guidelines are available for duct vent design. Indeed, strong dependence on geometry has been experimentally observed for the peak overpressure. As a consequence, the extension of semi-empirical equations is acceptable only as a rough estimation if “real” equipment are considered. Computation Fluid Dynamics (CFD) gives the opportunity of reproducing the complex phenomenon of venting for virtually any geometry, thus providing the tools for the comprehension of combustion mechanisms subtended to the pressure evolution in the equipment, and eventually supplying design guidelines for the specific case of ducted vent. To this aim, development and validation of the code is needed prior to any consideration. In this preliminary work, gas explosion in ducted vented vessels, recently presented in the literature, has been simulated by means of CFD-ACE+, once combustion sub-models for laminar, flamelet and distributed combustion regimes have been implemented. Results are discussed in order to confirm the assumption that the main cause of the increased severity of the overpressure in ducted vents is the vigorous burn-up in the duct and reverse back flow to the ignition vessel, whereas the hydromechanical drag in the duct and non-negligible purely mechanical pressure peak (secondary peak), due to the compression wave generated by the explosion in duct propagating backward in the vessel are not essential.