The main objective of this work is to study under ventilated fires. Simulations of a fire in a compartment containing a ventilation network are made with two fire modelling programs, MAGIC and CDI, in order to study the ignition risk of unburnt gases in the extraction vent. They show that an insulating material favours the ignition and that a fire with a low Heat Release Rate (HRR) produces few unburnt gases but with a high HRR, the fire is much shorter. An intermediate HRR will favour the ignition in the extraction vent. A comparison between the results obtained with the two programs is made. This study is a design work for some foreseen fire tests in an 8-cubic-meter compartment at the Institut P’, in France.