Pro-Form DVF6-WT 4" Inside 6 5/8" Fire Stop for Flex used with Pro-Form Direct Vent Systems. This fire stop is used for situations where a pipe must be ran vertical to the ceiling or floor to prevent combustion. The framing distances are for a 1" clearance - 9.5" x 9.5", for 2" clearance - 9.5" x 10.5", for 3" clearance - 9.5" x 11.5".