Natural draft opening
Natural draft opening means any opening in a room, building, or total enclosure that remains open during operation of the facility and that is not connected to a duct in which a fan is installed. The rate and direction of the natural draft through such an opening is a consequence of the difference in pressures on either side of the wall containing the opening.
40 CFR § 63.742
Terms used in this subpart are defined in the Act, in subpart A of this part, or in this section as follows: