If an attic fan is properly sized, and properly installed, what should be
the temperature differential between the outside air temperature and the
attic temperature, while the fan is running?
Assume there is plentiful air intake in the attic from outside air.
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Attic temperature depends on the amount of solar radiation, construction
details and the rate of ventilation. Calculations indicate that on a July
day in Texas, a ventilation rate of one air change per minute for a typical
attic using 95-degree F air will lower the peak attic temperature to about
101 degrees F. Providing half air change per minute will lower the
temperature to about 106 degrees F. Thus, the first half change per minute
is most effective and a doubling of this rate only achieves about 5 degrees
F additional cooling. Studies indicate that further increases in ventilation
are not effective in significantly reducing attic temperatures.
Calculate the required summer ventilation rate by determining the volume of
attic space and dividing by 2. This will be the cfm (cubic feet per minute)
of ventilation air needed. The volume is determined approximately for a
rectangular house by multiplying the height from the ceiling to the
peak/ridge (H) times the width of the house (W) times the length (L) and
dividing by 2 -- ( H x W x L / 2 ). For a gable roof, this will be
reasonably accurate. For a hip roof house, the volume will be overestimated
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