VBK-BGR Vent blockers are used to prevent the recirculation of hot air between top-mount fans and nearby vent openings. Vent blockers should be placed over the enclosure's vents located on the upper front of the enclosure, upper sides, and upper rear door (depending on enclosure). This will ensure that heated enclosure air will be forced out through the top-mounted exhaust fans instead of re-circulating, keeping equipment cool.