Buy Now Electrical Installation
Wiring of the attic fan unit should be done only by a competent electrician who is familiar with the local electrical code. For convenience, a switch for the attic fan should be located in the living quarters of the house. This may be a time switch, which will automatically turn the fan off a a pre-determined hour each night, or a simple manual switch.
If manual switching is to be used, in a two story house, use of a "three way" system is desirable, locating the switches on the first and second floors.
A thermostatic control or simple time switch may be used in a similar manner to turn off the fan during the night when the house has been cooled to the desired temperature.
When purchasing an attic fan unit it is advisable to select one having some type of overload protection on the motor. This overload switch will stop the motor in cases of overheating caused by the operation of the attic fan with the ceiling opening blocked off, or with the outlet louver closed by a winter panel.
Exhaust Openings
Table II was prepared to aid in determining the correct outlet area necessary for each standard size of attic fan. This table shows the minimum free area which must be available between the fan and outdoors for satisfactory operation. Extensive tests have shown that for best results the outlet area for an attic fan should be one and one-half times the area of the fan. Satisfactory results will, however, be obtained as long as the area is at least equal to the blade area. The table is a compromise between optimum outlet areas and ones of practical size.
Table II shows dimensions of square outlet louvers of different types of construction which are recommended for the standard sizes of attic fan. Construction features of these types of louvers can be seen on below.
This table also gives the dimensions for triangular wood slat outlet louvers, as shown below, which may be used with standard attic fans.
To determine the dimensions of the louver, it is necessary to know the pitch of the roof. Opposite the diameter of the fan you have selected you will find correct dimensions for the height of the triangular louver for roof pitches of 5, 6, 7 or 8 inches to the foot. With roof pitch and triangle height established, sides and base of the triangle are automatically correct.
When the attic fan unit is installed in an end wall, or in a window, the space between the fan unit and the outlet louver should be made airtight with some material that will also prevent vibration from being transmitted from the unit to the frame of the house. A strip of felt or rubber will be sufficient. If the front of the opening (in which the blade rotates) cannot be mounted within an inch or two of the outlet louver, a short canvas duct can be used to carry the air from the fan to the louver. This duct should be attached to the outer edges of the fan opening and to the edges of the outlet louver in order to keep its area considerably larger than that of the fan opening. All connections should be made as airtight as possible.
If the suction box type of installation is used it may be possible to locate the outlet opening under the eaves of the house or in a porch ceiling (if the attic extends over these areas.)
Generally speaking, outlet louvers should be constructed of metal if possible, as the shutters of metal louvers are thinner than those of comparable wooden louvers, and do not obstruct the flow of air as much. The result, as can be seen below, is that metal outlet louvers do not need as much overall area as do wooden louvers to provide the equivalent free area (the actual open area through which air can pass).
| Metal slats in exhaust louvers allow maximum area for air to pass | Wood slats in exhaust louvers are thicker, restrict air flow |
In order to keep birds, squirrels, etc., from entering the house through the outlet of the attic fan, a wire mesh screen should be placed across the interior face of all fixed louvers. On-half or one inch mesh wire is recommended. One inch mesh hardware cloth is figured as having 90% free area; on-half inch mesh as having 80% free area. If it is desired to make the attic proof against insects as well, this can be done by "boxing" the screen as shown below. Since conventional fly screening has but 50% free area this must have an area twice that of the inlet opening.
Frequently, architectural design considerations and the practical convenience of an automatic shutter are combined by mounting the automatic shutter back of the outlet louver.
How an exhaust louver is "boxed" with fly screen
Table II has been computed with an allowance for 1" mesh wire over the openings. No screening is necessary for automatic or motor driven shutters, as they close when the fan is not in operation.
In winter, installations having automatic or motor driven louvers may be used to exhaust smoke or cooking odors. Fixed grill installations, however, require installation of a winter panel over the outlet louver to keep cold air out of the attic, which would make fan operation impossible.
Table II
Recommended Dimensions of Attic Fan Outlet Louvers
| Fan Diameter |
Air Delivery Range CFM |
(1) Free Outlet Area Needed |
| 24" | 3500 | 4.70 sq. ft. |
| 5000 | ||
| 30" | 4500 | 7.35 sq. ft. |
| 8500 | ||
| 36" | 8000 | 10.06 sq. ft. |
| 12000 | ||
| 42" | 10000 | 14.40 sq. ft. |
| 15500 | ||
| 48" | 12000 | 18.85 sq. ft. |
| 20000 |
| Fan Diameter |
(2)Dimensions For Square Outlets |
||
| (3)Metal Automatic | (4)Metal Fixed | (5)Wood Slat Fixed | |
| 24" | 26 x 26 | 32 x 32 | 34 x 34 |
| 30" | 32 x 32 | 39 x 39 | 42 x 42 |
| 36" | 38 x 38 | 45 x 45 | 49 x 49 |
| 42" | 44 x 44 | 54 x 54 | 60 x 60 |
| 48" | 50 x 50 | 62 x 62 | 68 x 68 |
(2) Computed to include 1" Mesh Wire Screen over Metal Louvers - If Wood Slat Louvers are installed use next largest height shown
(3) 90% Open Area
(4) 70% Open Area
(5) 60% Open Area
| Fan Diameter | Height of Triangular Louvers (For Different Roof Pitches) | |||
| 5/12 Pitch | 6/12/Pitch | |||
| One Louver | Two Louvers | One Louver | Two Louvers | |
| 24" | 2' 0" | 1' 8" | 2' 2" | 1' 10" |
| 30" | 2' 6" | 2' 1" | 2' 9" | 2' 4" |
| 36" | 3' 0" | 2' 6" | 3' 3" | 2' 9" |
| 42" | 3' 3" | 3' 0" | 3' 9" | 3' 3" |
| 48" | 3' 10" | 3' 4" | 4' 3" | 3' 7" |
| Fan Diameter | Height of Triangular Louvers (For Different Roof Pitches) | |||
| 7/12 Pitch | 8/12 Pitch | |||
| One Louver | Two Louvers | One Louver | Two Louvers | |
| 24" | 2' 4" | 2' 0" | 2' 6" | 2' 1" |
| 30" | 3' 0" | 2' 6" | 3' 3" | 2' 8" |
| 36" | 3' 6" | 3' 0" | 3' 9" | 3' 4" |
| 42" | 4' 1" | 3' 6" | 4' 4" | 3' 9" |
| 48" | 4' 7" | 4' 0" | 4' 9" | 4' 3" |