Previous studies have indicated that the expansion of fire seals and fire stoppers will be dependent on the heat exposure. Standardized methods for classification includes exposure to a rapid heat exposure of the product that is tested, but this might not always be the worst case scenario for the product. In this paper a series of tests are presented in order to study how a graphite based intumescing system, intended for cavities, reacts when subjected to fire conditions different form that in the standardised method EN 1366-4. Two different experimental setups, a cone calorimeter at Lund University and a small-scale furnace at SP Fire Technology, were used in the tests. The start of expansion reaction in the tested fire stopper was around 180°C. The start of reaction temperature is rather independent of the incident heat flux, while expansion rate is clearly dependent on the incident heat flux. Furthermore, the studied fire stopper performed well in the small-scale furnace because the graphite expanded enough to give the same protection, although it is preheated or wet, as when subjected to a heat exposure similar to ISO 834. These results are good but they are only considered valid for the studied system and for the given situation. Future research is needed in order to study how other types of intumescing fire protection systems work when subjected to realistic fire environments.