page 5 of 16
VENTILATION REQUIREMENTS
It is very important that the attic be suffi
ciently ventilated for the fan system to operate properly. Without adequate
ventilation, hot air exhausted from the home cannot easily escape from the attic, which creates back-pressure that
will substantially reduce the fan’s performance. Operating this fan in an attic with less net free ventilation area
than recommended will decrease its airfl ow and energy effi
ciency.
If subject to the California Title 24 requirements for whole-house fan ventilation, the fan system must be installed to
the minimum ventilation requirements as found in the standard.
In the absence of applicable local code regarding whole-house ventilation, we recommend a minimum of 1 square
foot of “net free” ventilation area per 500 cfm at a fan’s highest speed for proper operation. Refer to specifi cation
Refer to specifi cation
table for required Net Free Ventilation Area in Attic at HVI fan rating.
table for required Net Free Ventilation Area in Attic at HVI fan rating.
Net
Net Free Ventilation Area
Free Ventilation Area can be provided by any combination of gable, eyebrow, roof cap, soffi
t, or ridge
vents, or any other method of ventilating the attic space. The openings of most vents are partially obstructed by
grilles, louvers, and/or screens. A vent’s “net free” ventilation area is the surface area of its opening minus the surface
area of any grilles, louvers, or screening covering it. Diff erent types of vents have diff erent ratios of net free area to
total area.
While most properly constructed homes have adequately ventilated attics, not all do. Because suffi
cient ventilation
is a very strong factor in this fan’s performance, it is important that the home’s existing ventilation be verifi ed before
it is installed.
Manufacturers typically publish their vents’ net free ventilation areas and/or ratios in their products’
specifi cation documents. If this information is unavailable, a ratio of 50% net free area to total area is usually a good
rule of thumb. A notable exception to this rule of thumb are ridge vents. The industry standard net free ventilation
area for ridge vents is 13% of the vent’s length in feet.
Since most attics have multiple vents, often of diff erent types, it is necessary to count each vent, noting its type
and size. Apply the appropriate ratio to the dimensions of each vent to fi nd its net free area, and sum these
values to fi nd the attic’s total ventilation. An example of how these calculations are made is given in the table below:
Vent Type
Dimensions
Total Area
Net Free Area Ratio
(“NFA”)
Net Free Ventilation Area
(=Total Area x NFA)
Two Louvers
24” x 24”
24” x 24” / 144 = 4 ft.
2
50%
2 x 4 ft.
2
x .50 = 4 ft.
2
Ridge
40 feet
n/a
13%
40 feet x .13 = 5.32 ft.
2
Round Soffi
t 10” diameter
3.14 x 5” x 5” / 144 = .55 ft.
2
50%
.55 ft.
2
x .50 = 0.28 ft.
2
Total Net Free Ventilation Area:
9.60 ft.
2
Please consult a roofi ng professional if the attic’s net free ventilation area remains uncertain. Additional ventilation may be
required for proper operation of the fan system.
P/N 780105002 01/21 Rev A
Refer to specifi cation table for required minimum Open Window Ventilation Area in square feet when operating
Refer to specifi cation table for required minimum Open Window Ventilation Area in square feet when operating
whole house fan. Without adequate Open Window Ventilation Area, the quantity of cooler air, pulled into the home
whole house fan. Without adequate Open Window Ventilation Area, the quantity of cooler air, pulled into the home
to be exhausted to the attic, may substantially reduce the fans overall performance.
to be exhausted to the attic, may substantially reduce the fans overall performance.
