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VENTILATION REQUIREMENTS
It is
critical
that the attic be suffi ciently ventilated for this fan to operate properly. Without adequate ventilation,
hot air cannot easily escape from the attic, which creates back-pressure that can substantially reduce the fan’s
performance. Venting requirements vary by fan. We recommend
a minimum
of 1 sq. ft. of “net free” ventila-
tion area per 500 cfm at a fan’s highest speed.
Therefore, the VentCool 1.7™ Whole House Fan requires a
Therefore, the VentCool 1.7™ Whole House Fan requires a
minimum of 3.4 square feet of net free ventilation area for proper operation.
minimum of 3.4 square feet of net free ventilation area for proper operation.
Operating this fan in an attic with less net free ventilation area than recommended will decrease its airfl ow and en-
ergy effi ciency. Net 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 then the surface area of its opening minus the surface area of any grilles, louvers, or screening covering it.
Different types of vents have different ratios of net free area to total area. Manufacturers typically publish these ra-
tios with their vents’ specifi cations, but if this information is not available to you, a ratio of 50% net free area to total
area is usually a good rule of thumb. For example, according to this rule of thumb, a typical 24” x 24” louver with
a gross area of 4 sq. ft. would have a net free area of 2 sq. ft. A notable exception to this rule are ridge vents. The
industry standard net free ventilation area for ridge vents is 13% of the vent’s length in feet.
While in our experience most properly constructed homes have adequately ventilated attics, not all do. Because
suffi cient ventilation is so critical to this fan’s performance, it is important that the home’s existing ventilation
be verifi ed before it is installed. Since most attics have multiple vents, often of different types, it is necessary
to count each vent, noting its type and s ize. Then, apply the appropriate ratio to 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 TABLE 1 below:
Please consult a roofi ng professional if the attic’s net free ventilation area remains uncertain.
Vent Type
Vent Type
Dimensions
Dimensions
Total Area
Total Area
Net Free Area
Net Free Area
Ratio (“NFA”)
Ratio (“NFA”)
Net Free Ventilation Area
Net Free Ventilation Area
(Total Area x NFA)
(Total Area x NFA)
Louver
24” x 24”
24” x 24” / 144 = 4 ft²
50%
4 ft² x 0.50 = 2 ft²
Ridge
10 feet
n/a
13%
10 feet x 0.13 = 1.33 ft²
Round Soffi t
10” diameter 3.14 x 5” x 5” / 144 = .55 ft²
50%
0.55 ft² x 0.50 = 0.28 ft²
Total Net Free Ventilation Area =
3.61 ft
²
WHERE TO LOCATE THIS FAN
The best location for this fan is dictated by its theory of
operation: As a home heats up during the day, a large amount of
heat is retained in its structure and contents. These materials give
up their heat slowly and, in doing so, continue to heat the home’s
interior even though the outdoor temperature may, in fact, be very
comfortable in the evening and at night. Thus, homeowners are
forced to either endure the hot conditions inside of their homes or
turn on their air conditioners and bear the expense thereof.
When operated properly, this whole house fan can resolve this
dilemma by forcing the hot air inside a home out and drawing cool
air from outside in. The illustration, to the right, depicts how this fan
exhausts hot into the attic and draws cool air into the house from
outdoors through open windows and/or doors.
TABLE 1: Net Free Ventilation Calculation
TABLE 1: Net Free Ventilation Calculation
P/N 78010008000 05/19 Rev B
