A computer system includes an enclosure, a backplane within the enclosure, a replaceable unit releasably connected to the backplane, an air cooling system configured to create an air flow along the replaceable unit and an air flow blocking system. The air flow blocking system includes a first member coupled to the enclosure and a second member coupled to the replaceable unit. The first member and the second member interact to at least partially block air flow along the replaceable unit based upon at least one characteristic of the replaceable unit.