Installation Methods
After the air has been drawn through the house and up into the attic through the ceiling opening it is drawn through the fan and exhausted outside of the house through an outlet opening. A number of different types of louvers may be used in this outlet opening, the selection depending on the general house construction and on the number and type of outlet openings that may already exist.
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It is recommended, however, that the fan be located so that it will not exhaust air out the side of the house which faces the prevailing winds. If the fan must blow air out against the wind it will be operating against additional resistance, and air delivery will be slightly reduced. The following locations may be used for installation of attic fans.
If there are no open spaces under the eaves, and the attic is in other respects reasonably air-tight, so that the fan will draw all its air from the house through the ceiling opening, and not directly from outdoors through these other openings, there are four choices as to how the installation can be made:
(1) In a house having a suitably sized window in an attic end wall or dormer, mounting an attic fan against such a window, presents the fewest construction problems. The window can be opened and the fan mounted in this opening. This type of mounting should also include louvers a few inches in front of the fan to keep rain out of the attic, though this is not absolutely necessary as long as the window is closed whenever the fan is not being operated.
Horizontal discharge fan mounted in gable end; ceiling grill in central hallway
A louver of wood or metal may be used or an automatic type of shutter may be installed. There shutters are made by other manufacturers and are sold by Electric Trading Co. When the fan starts, the shutter is automatically opened by air pressure. When the fan stops, the shutter closes.
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(2) If the house has no attic windows, or if the fan required is too large to be mounted in the window available, an opening can be cut in the end wall of the attic big enough to accommodate the unit and its outlet louver. If an opening must be cut through the end wall, it should be framed just like a window sash.
Vertical discharge fan mounted above ceiling grill, twin exhaust louvers
(3) Houses having extremely low attics may not have sufficient height to permit the mounting of a horizontal discharge unit through an end wall. If such a case a vertical discharge unit may usually be mounted directly over the ceiling opening. Vertical discharge units are similar to standard horizontal discharge attic fans, but are equipped with motors which can be operated in a vertical position. Since they have a minimum of depth, vertical discharge units can be successfully installed in most low attics.
However, the distance between the fan blade of the unit and the nearest portion of the roof must be a least equal to the diameter of the fan blade. Operation at closer distances will result in excessive air noise and an increased load on the fan's motor.
(4) If there is not sufficient clearance for a vertical discharge unit it may be necessary to construct a dormer of sufficient size to house the proper horizontal discharge unit. The height and width of the dormer will be determined by the dimensions of the attic fan and of fans in dormers discussed later.
In houses having attics with open spaces under the eaves, or otherwise not airtight, there are three commonly used types of installations. In each, the installation should be made directly over the ceiling opening. Either a vertical or a horizontal discharge unit may be installed.
Horizontal discharge fan mounted in suction box over ceiling grill; twin exhaust louvers
(1) The horizontal discharge unit will require construction of a suction box to insure that the fan will exhaust air only from the rooms below rather than draw air into the attic from the open spaces under the eaves, etc.
In installations which include a ceiling opening into living quarters below provision should be made for closing the opening during cold weather. Numerous trapdoor arrangements are possible, but the best treatment of the problem is the installation of an automatic shutter. Such shutters are readily available, and are effective. They open automatically whenever the fan is started, and close when it is shut off, sealing off the attic from the rooms below.
(2) If there is an existing stairway to the attic, use a horizontal discharge type unit mounted in a suction box built around the stair-head, with a door in the end of the suction box for access to the attic is the most satisfactory means of drawing air into the attic.
(3) In houses with hip roof construction, either a vertical or horizontal discharge unit may be installed directly over the ceiling grill. Provision of "strip louver".....a narrow slot in the soffit of the cornice all around the house is an excellent, pleasing and inconspicuous means of providing exhaust area.
Size of Ceiling Opening
The ceiling opening, and its accompanying grill or shutter must be of a size that will not cause excessive resistance to air flow, and will allow the air stream to pass through at a moderate, quiet velocity. The area of the ceiling grill determines both these factors.
Therefore, determination of the size of the ceiling opening is the next step. The area needed must be shaped to the physical dimensions of the ceiling, since a narrow hallway may require a rectangular opening and a large hall may make a square opening equally practical. Even ceiling joist spacing may be a factor, since it is wiser to elongate the opening slightly than to cut out another joist to gain an inch or two of width.
Preparing Ceiling Opening
Typical framing for ceiling grill or installation of vertical discharge fan
Be sure that the area selected for the ceiling opening is not crossed by plumbing, electrical lines or other obstructions. Then proceed to cut an opening four inches greater in each dimension than is actually required for inlet area or for insertion of the automatic shutter if one is to be installed. This additional area provides room for insertion of short lengths of 2" wide material, or "headers" to restore the structural strength of the ceiling.
At this point the rough opening offers an easy means of getting the attic fan unit into the attic. Finish lumber may then be applied around the opening, if desired and allowed for and the automatic shutter or grill fastened in place and trim applied to cover the crevice between grill and ceiling.
It should be noted here that in any case where the home owner prefers a decorative metal grill for appearance sake, yet likes the convenience of the automatic shutter, that both may be used by positioning the shutter two inches or more higher inside the opening.
Installation of the attic fan may not be completed before proceeding to make the exhaust opening. In fact, the moment the opening has been cut, the fan may be used to cool the attic and makes possible the finishing of the opening in greater comfort.
Non-Rigid Mounting Recommended
Although an attic fan is inherently quiet, due to its low speed and carefully balanced blades, there may be some slight vibration of the unit. If the unit were rigidly mounted to the framework of a house, this vibration could be amplified by the wall and ceiling panels (which act as sounding boards) until it became objectionable.
The fan unit should, therefore, be hung on springs from the rafters or mounted on rubber, felt, cork or some other sound deadening material which will absorb rather than transmit vibration. Small blocks of this material one inch or more in thickness should be sufficient if placed under each corner of the unit.
If a vertical discharge type of unit has been used, setting the fan in position over the ceiling opening completes that phase of the work, and electrical connections may be made at once. The illustration below shows the details of a vertical installation.
Detail of a vertical discharge fan installation
The subject of suction boxes for horizontal discharge fans will be treated later.