Updraft ventilation (overhead hood), collects and removes cooking vapors several feet above the cooking surface. Because cooking vapors naturally rise, updraft systems tend to be the most effective method of ventilation. Downdraft systems provide an alternate method of providing effective ventilation, for non-professional cooktops and ranges. Smoke extraction is designated as natural smoke extraction (NRA) if it works via natural thermal lift, so without additional ventilation drives (such as ventilators). Natural smoke and heat extraction units (NRWG) include, for example, windows, skylights, ventilation blinds which automatically open in the event of a fire and help draw smoke and ... Jan 26, 2011 · These openings provide sufficient ventilation for both smoke clearance and to remove exhaust fumes. The 2006 edition of ADF now defines the area of wall openings as an "equivalent area", meaning designers have to take full account of the aerodynamic effect of any louvres, screens, barriers, etc across the ventilation openings.