For an installation using natural
ventilation, you should consider the location
and control of ventilation openings. Locate
the air inlet openings on the side of the
building facing directly into the prevailing
winds. Locate the air outlets where prevailing
winds movements would create low-pressure
areas; that is, on the side directly opposite the
prevailing wind direction. Outlets may also be
placed on a roof in the form of individual
gravity ventilators or ridge ventilators.
Calculate the ventilation rate due to wind
velocity and the stack effect as detailed in
criteria established by the American Society of
Heating, Refrigerating, and Air-conditioning
Engineers (ASHRAE). When natural
ventilation is provided for temperature control,
you should provide a means for closing the
openings during the heating season.
The use of gravity ventilators is another
method. A roof-mounted gravity ventilator
may be a stationary, a pivoting wind-
directional, or a rotating-turbine type of
ventilator. You should select gravity
ventilators based on the rating tables for the
mounting height involved and a wind velocity
of 4 miles per hour.
Natural ventilation has limited uses. In
general, natural ventilation is inadequate for
the following examples:
Offices having an open window area less
than 5 percent of the floor area
Offices over 24 feet deep and without
Offices having cross ventilation but
having occupied space more than 35 feet from
a window or an air inlet
Dining rooms having a window area less
than 6 percent of the floor area
In using natural ventilation, you should
consider local building and safety codes and the
minimum requirements of the Occupational Safety
and Health Act (OSHA), part 1910.
Mechanical ventilation uses mechanical forces
to induce air circulation within buildings or spaces.
Air movement is created by fans or by fans
combined with a supply air and/or exhaust air duct
You should provide mechanical ventilation
equipment when the necessary quantity of outside
air cannot be supplied continuously by natural
forces. The quantity of air supplied should be kept
to an acceptable minimum. You should install
mechanical ventilation equipment in the following
For a supply of outside air and the removal of
bad air or air contaminated by smoke, body odors,
and so forth, in areas having a high occupancy
level (auditoriums, assembly halls, and cafeterias).
For processes giving off noxious or hazardous
fumes, dust, or vapor, resulting in unsafe or
unhygienic conditions (paint spray booths,
electroplating plants, welding booths, and other
For limited comfort of operators as in
laundries, projection booths, and kitchens.
For spaces containing fumes and vapor with
specific gravity higher than air, such as garages
and some refrigeration rooms. In these cases,
provide exhaust intakes at floor level.
For electronic or electric equipment installed in
confined spaces where the operating temperatures
of the equipment may exceed the safe limit.
For spaces having explosive vapors or dust,
use explosionproof ventilation equipment
regardless of the concentration of explosive
For odor removal in bathrooms.