Ignition risk of unburnt gases in the extraction duct of an underventilated compartment fire was studied from 8-cubic-meter room fire at the Laboratoire de Combustion et de Detonique, in France. A study of factors that have an influence on the ignition risk at the extraction is made. Two main factors appear: heat release rate and ventilation flow. These factors were studied by changing ventilation flow for different diameters of fire source. Fire tests were also made in order to understand the impact of closing the inlet vent on the production of unburnt gases. Criteria of heat release rate and ventilation flow are determined in order to predict conditions that lead to an ignition risk.