Suction Boxes
Dimensions of suction boxes for 24" through 48" attic fan units are given in Table II. The "A" dimensions given are not critical. Usual construction procedure is to make the height and width of the box to conform to the outside dimensions of the attic fan unit. Remember, however, that part of the suction box will be directly over the ceiling opening, and its width must be at least as great as the smallest dimension of the opening.
The suction box can be constructed of masonite, or any sound absorbent wall board nailed to a reinforcing frame of 1" x 2" lumber. The framing should be placed on the outside of the box as shown below, in order to have the smoothest possible interior. If the framing were on the inside it would cause turbulence of the air flow, and noise might possible result. Sides and top of the box should be stiffened with 1" x 2" material as found necessary to prevent sagging and unnecessary vibration of these panels.
Detail of horizontal discharge fan installation with suction box, including fusible link for grill cover.
The inside of the suction box may be painted black so that it will not be visible from below the ceiling opening.
In order to prevent transmission of vibration from the fan unit to the suction box, a strip of canvas two or three inches wide should be used to connect the two, as shown. Connections between the canvas, fan unit and suction box should be airtight.
In some locations, local fire codes require that an automatic fire door be installed in a suction box. The illustration shows a simple method of installing a trap door connected to a 135 degree fusible link. In case of a fire that fusible link will melt, the fire door will close and the fan will stop. When the fan is not being operated, this trap may be closed to seal off the attic from the house.
If there is a chance of anyone being near the attic fan when it is operating, install a wire mesh guard before the air intake side of the unit to prevent close contact with the moving parts of the unit.
One fan unit is usually used in a suction box installation. If very high air delivery is required and insufficient clearance is available for the installation of a large-sized unit, two smaller units may be used whose combined output equals the desired volume of air. The two units should be of the same type and size. The should be mounted in opposite ends of the suction box, equally spaced from the ceiling opening.
Dimensions of the ceiling opening should of course be based on the combined air deliveries of the two fans.
If the attic has pellet or granular type insulation between the joists, a strip of building paper should be nailed in front of the suction box so that the insulation will be not blown out of place.
Motor driven shutters in the outlet louvers are often preferred when suction boxes are used, and when the outlet is located at some distance from the suction box. These shutters have an electrically driven motor connected to the vanes. This motor is connected in the same circuit as the motor of the attic fan. When the fan is started the shutters are opened and remain open until the fan is stopped, when they are automatically closed again.
It is desirable to face the opening in the suction box away from the outlet louver in order to create the maximum possible air circulation through all parts of the attic. The outlet louver should be placed a s far from the suction box as is convenient.
Since vertical discharge units and suction box installations actually create a small amount of pressure in the attic, as opposed to other types of mounting which create a mild vacuum, several outlet louvers, located wherever desired may be used.
Suction Box Dimensions
| Fan Diameter |
Dimensions |
|||
| A | B | C | D | |
| 24" | 32" | 40" | 24" | 64" |
| 30" | 38" | 48" | 24" | 72" |
| 36" | 44" | 56" | 24" | 80" |
| 42" | 50" | 64" | 24" | 88" |
| 48" | 56" | 72" | 24" | 96" |
Dormer Installation
When clearance for installation of a horizontal or a vertical discharge unit is not available, a dormer may be constructed, or a penthouse added to the roof of buildings such as apartment houses, which have flat roofs.
The style and location of the dormer should be such as to match with the exterior of the house. Its size should be determined by the size of the outlet louver necessary and of the unit itself. The sides and top of the unit should be insulated from the dormer with rubber, felt, etc., to prevent transmission of vibration and should be made air-tight. Automatic or motor driven shutters are preferred with dormer construction, so that the fan will not have to be removed in the fall to allow installation of a winter panel.
The fan should not discharge air directly at a wall, chimney, etc. A distance of 6 to 8 feet should be maintained between the fan and any such obstruction.
Operation and Maintenance
The major point that must be remembered is to open a door or window before the fan is turned on. Otherwise, if there is a fireplace, air will be drawn down the chimney along with any loose soot, and this dirty air will be circulated throughout the house.
Once installed, an attic fan should require a minimum of maintenance, and should give many years of dependable service.
As a precautionary measure, the fan should be inspected occasionally during its operating season. Points to be checked are:
(1) Lubrication instructions on the motor should be followed.
(2) Bearings through which the fan shaft passes should be lubricated in accordance with the manufacturer's recommendations.
(3) Check drive belt for frayed edges or other signs of wear. Replace belt when necessary. Check belt for tension - adjust if necessary. A V belt should be tightened just enough to stop slippage.
(4) Once a year wipe dust, dirt and spilled oil from entire unit, including the fan blades. Use care in cleaning the blades, as they have been accurately aligned and balanced at the factory, and bending will cause serious vibration.
This inspection should be made only after the unit has been disconnected form the electric mains. This can be done by removing the fuse in the attic fan circuit, or by throwing the switch if the fan has its own fuse box.
The information presented in this instructional should be sufficient to cover most problems encountered in any normal attic fan installation.