for each primary occupied space to maintain comfort during periods of low
windows may be shut. Refer to para. 3.1.4e.
Whole-House Fans. In some cases wind-driven natural ventilation
through open windows may not provide sufficient ventilation to exhaust heat
from the building's interior. Constrained building orientation or dense
surroundings may prevent the wind from creating pressures across the building.
In such cases whole-house fans, which typically induce 30 to 60 air changes
per hour, may be necessary as backup units. Whole-house fans have low initial
investment costs (about 0 to 0 installed) and low energy use (between
300 and 500 watts, roughly one tenth the consumption of an air conditioner).
The whole-house fan
operates by pulls air in through
open windows and
Figure 42). Openings in the floor
are sometimes used to draw air from
the cooler, shaded underside of an
elevated building. A whole-house
fan should be centrally located in
the building, above a public area
such as a hall or stairwell, so that
it draws in air from all parts of
Whole-house fans are
primarily used for cooling the
building's structure, often by
enhancing night ventilation. The
fan is turned on when the outdoor temperatures drop in the late afternoon or
early evening. In the morning, the fan should be turned off and the windows
closed before the outdoor temperatures begin to rise above the interior
Sizing of Openings for Whole-House Fans. The total open window
area should be approximately two times the open area of the fan. The total
open window area should be three times the whole-house fan open area if there
is insect screening at the windows. It is not necessary to open windows all
the way to ventilate with a whole-house fan. They can be opened 4-6 in.
(100-150 mm) and fixed in a secure position by stops or window locks.
The attic vents need to be larger than normal for effective whole-
house fan ventilation. The free exhaust area should be approximately twice
that of the area of the fan itself, and three times the area if screening is
used. Openings should be distributed throughout the attic or placed to the
lee side of the building for adequate ventilation. Refer to Appendix C,
Section 4 for whole-house fan sizing procedure.
Fans for Body Cooling. Although whole-house fans can create some
air motion, especially near windows and near the fan outlet, the interior
velocities created are in general too low for body cooling. Therefore, ceiling
fans or portable oscillating fans are recommended for body cooling. It is
possible to use both types of fans in combination in one building.