Instructions / Assembly

For proper operation of your Comfort Cool whole house fan, it is CRITICAL that your attic has

sufficient venting area; otherwise the hot air cannot easily escape and creates back-pressure that

can substantially reduce the performance of your new whole house fan.

The “net-free” area of a vent is the total vent opening minus the loss caused by the interference

of the screen, louver or grille covering the vent.

 The Centric Air 1.5 requires a MINIMUM of 1.5 sq. ft. of "net free" venting area in your

attic to allow for proper exhaust. This means that it requires the equivalent of a 2 ft. by 2 ft.

unobstructed hole.

 The Centric Air 2.0 requires a MINIMUM of 2.5 sq. ft. of "net free" venting area in your

attic to allow for proper exhaust. This means that it requires the equivalent of a 2 ft. by 2 ft.

unobstructed hole.

 The Centric Air 2.7 requires a MINIMUM of 3.25 sq. ft. of "net free" venting area in your

attic to allow for proper exhaust. This means that it requires the equivalent of a 2 ft. by 2½ ft.

unobstructed hole.

 The Centric Air 3.1 requires a MINIMUM of 4 sq. ft. of "net free" venting area in your attic

to allow for proper exhaust. This means that it requires the equivalent of a 2 ft. by 3 ft.

unobstructed hole.

 The Centric Air 4.0 requires a MINIMUM of 5.25 sq. ft. of "net free" venting area in your

attic to allow for proper exhaust. This means that it requires the equivalent of a 2 ft. by 3 ft.

unobstructed hole.

Since most attics have multiple vents, often of different types, and since most vents are partially

obstructed by grilles and/or bug/animal screens, you’ll need to do some calculations to make sure

your venting is sufficient. While it is our experience that most properly constructed houses have

the required venting, not all do. And because this is so critical to the proper operation of your unit,

it is important that you verify it. Different types of vent designs have different ratios of obstruction

caused by grilles and screening and manufacturers typically publish these numbers. If this

information is not available to you, a ratio of 50% is a good rule of thumb. For example a typical

24" x 24" louver, with a gross area of 4 sq. ft. would have a net free area of 2 sq. ft.

Vent Length Width Net Free Calculation

Type Area (NFA%) L x W x NFA / 144

Louver 16” 16” 50 % = 16 x 16 x 0.5 /144 = 0.89 sq. ft.

Ridge Vent 48” not used 13 % = 48 x 0.13 / 12 = 0.52 sq. ft.

Eave Vent 12” 4” 50 % = 12 x 4 x 0.5 / 144 = 0.16 sq. ft.

 In practice, less net-free area than is recommended will decrease the airflow performance of

the unit.

 If you are unsure as to how much net-free venting you have, please consult a roofing

professional.

Net-free venting area can be acquired by any combination of gable, eyebrow, soffit, or ridge

vents, or any other means that provide ventilation to the attic space.

UIRED VENTING AREA