This paper deals with the consequences of applying ventilation procedures during confined fires. If the air intake duct is closed after the beginning of the fire, another risk may appear, namely the ignition of unburnt gases in extraction ducts. This configuration is typical of many industrial installations equipped with ventilation networks. Heptane and dodecane pool fires were performed in a reduced-scale compartment equipped with a mechanical ventilation network. Heat release rate, temperatures and unburnt species concentration were measured for different pan diameters, different ventilation flows, with and without closing the intake duct, to study fire development and the potential risk of ignition in the exhaust system. Empirical correlations to estimate unburnt species concentrations are given. Based on these correlations, lower flammability limit calculations and scaling laws, a risk assessment methodology is proposed and a decision support tool is provided. Experimental results clearly show that the auto-ignition of unburnt gases can happen near the extraction duct, with and without closing the intake duct. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.