Stadium BM720 Anti-Draught Black Hole® Airbrick Ventilator – 70cm² 21kW Solid & Cavity Walls Rustic Red Stadium BM720 black hole® anti-draught Airbrick Ventilator. 70cm² free area. 21kw nett rated input. two-part cavity liner with Central Joint and cavity drip - variable between 290mm and 390mm for both solid and cavity wall construction. Reversable for either high or low-level installation. Rustic Red Airbrick. White internal grille with screw caps. Light and Noise Blocking. Airbrick dimensions - 216 x 152mm. Grille dimensions - 290 x 200mm. The BM720 is the larger liner version of stadium's black hole® airbrick range - the BM701-3 uses a three part liner (230–375mm), primarily for cavity wall Installations whereas the BM720 has a two part liner with Central Joint and cavity drip, variable between 290mm and 390mm for both solid an