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How to Install an Attic Fan


Part 1 - Go to Attic Fans Part 1 

Why An Attic Fan Should Be Installed
What is An Attic Fan
Proper Planning Reduces Costs
How to Determine Size Required
How Attic Fan Capacity is Rated
Where to Locate the Attic Fan

Part 2 - Go to Attic Fans Part 2

Installation Methods
Size of Ceiling Opening
Preparing Ceiling Opening
Non-Rigid Mounting Recommended

Part 3 - Go to Attic Fans Part 3

Electrical Installation
Exhaust Openings

Part 4 - Go to Attic Fans Part 4

Suction Boxes
Dormer Installation
Operation and Maintenance

Exhaust Fans - Panel Mounted - Panel Mounted
Exhaust Fans - with Safety Guard


Why An Attic Fan Should Be Installed

     Why ventilate the attic?  For very good reasons!  The hot summer sun, beating down on the roof of the average home, makes a veritable furnace of the space under the roof.  Unless some means of dissipating this heat is provided, it radiates down through the  ceiling into the rooms below.  Long after the sun has set and outside temperatures have dropped many degrees, these rooms continue to receive this accumulation of heat.

     The dark, composition roof coverings used on over two-thirds of the homes built today absorb a greater amount of the sun's heat than did the wood shingles used in the past century.  While insulation retards the downward flow of this heat, it cannot prevent uncomfortable temperatures in the rooms below.  And unfortunately, though insulation is an asset to every home, it is as effective at holding heat inside the home in summer as it is in winter.  Natural ventilation of the attic space cannot keep pace with the heat load being absorbed by the roof.

     As summer progresses, there is an increasing accumulation of heat in the roof, beams, attic, walls and furnishings of the home.  While it is true that some lowering of temperature of the home occurs during the night, summer nights are too short to allow for anything like an equalization of temperature.

     Therefore, forced ventilation!  An attic fan or whole house fan provide a ready means of dispelling this heat, and not only that, it draws gentle breezes throughout the house whenever and wherever the home owner wants them.  By the simple touch of a button, cooling breezes are wafted in through the windows, sweeping the heat and stagnant air up through the attic and out of the house.  With an attic fan, the calm breezeless nights which weather bureau records prove to be the rule in many sections of the country, hold no threat of discomfort, no risk of tossing, sleepless nights.  

More or less ventilation for the rooms of the average home is a simple matter of opening or closing doors and windows.  At left, the entire house is being ventilated.  At right, the closing of a hall door has made the entire capacity of the fan available for cooling the bedrooms.

What Is An Attic Fan?

     The phrase "central heating" is commonly heard and understood.  An attic fan is basically a central ventilating system having three major components.

     First comes the unit itself.  This unit consists of the fan blade and its electric driving motor; an opening in which the blade revolves at slow, quiet speeds, and the necessary framework, shaft and bearings for mounting these parts.  Fan units are made for both horizontal and vertical discharge, and will be described.

     Since the fan unit is usually located in the attic and it is desired to draw fresh air into the house through the doors and windows in the lower floors or basement, the second essential is an opening between the living quarters of the house and the attic, through which the fan can draw this cooling air.  This opening is usually located in the ceiling of the upper floor of the house, often directly under the fan unit in the attic.  A grill which will match with the interior trim of the house or a suitably framed automatic shutter should be inserted in this opening.

     Once the fan has drawn air through the house and up into the attic it must exhaust this air to the outdoors.  If there are not sufficient ventilating louvers or windows already in the attic walls, an outlet louver of some type must be provided.

     When the attic fan is in operation, fresh air enters through any door or window which is opened.  Therefore, the entire house may be ventilated equally, certain rooms may be closed off from ventilation, or ventilation may be concentrated in one or two rooms depending upon which windows and doors are opened or left closed.  For example, in extremely hot weather it is often desirable to have the fan draw all its air through the one or two most used rooms in order to create a strong cooling breeze, rather than have a gentle circulation of air thought the entire house.

     During the winter months, the attic fan may be operated when needed to exhaust cigarette smoke or unpleasant cooking odors from the house.  One or two minutes operation are all that is necessary to completely freshen the air throughout the house without chilling.

Proper Planning Reduces Costs

     The cost of an attic fan installation will naturally vary with the size of the house and the type of installation required for the particular structure.  The cost is very moderate, however, when the resulting years of personal comfort are considered.  When an architect's services are obtained in planning a new home, provision can easily be made for the installation of an attic fan, either at the time the house is built, or at any later date.  Provision for an attic fan will entail little or no expense at the time the house is constructed, and will eliminate the need for any structural changes when the fan is installed.  

How to Determine Size Required

     When planning an attic fan installation, the first consideration should be the amount of air which will be needed to properly cool the house.  The quantity necessary will depend on the size of the house, the climate of the area and even the topography in the immediate vicinity of the house.  The absence of night time breezes or the existence of obstacles to the natural flow of wind currents will increase the amount of ventilation capacity that should be provided.  No exact rule can be made governing allowances for the variables of climate and topography.  Reason and experience must dictate these allowances, after cubical content of the house had been determined.  To do this, multiply the floor area of the living quarters of the house by the height from floor to ceiling.  For houses having more than one floor the volume of each floor should be computed and added together.  Do not include cellars, closets, closed off garages or porches in this computation, since air will not be circulated through these areas.

    Table I, below, will allow a quick approximation of the air-moving capacity which will be required for comfort, and indicates the fan diameter.

Table 1

Approx. Vol of House in Cu. Ft. Minimum Fan Capacity Needed For Satisfactory Results (in CFM) Residential Locations
Mild Climate Hot Climate Hot & Humid Climate
3000   1000     2000   24" 3000  
4000   1320     2640   4000  
5000   1650     3300   5000 30"
6000   2000   24" 4000     6000
7000   2310   4620 30"   7000
8000   2540   5280 36" 8000
9000   3000     6000 9000  
10000   3330     6660 10000 42"
11000 24" 3630     7260 11000
12000 4000   36" 8000 12000
13000 4290   8580 48" 13000
14000 4620 30" 9240   14000
15000 5000 10000 42" 15000
16000 5280 10560 16000  
17000   5610 11220 17000  
18000   6000 12000 18000  
19000   6270   12540 19000  
20000   6660   13320 20000  
21000   7000   14000 21000  
22000   7260   14520 22000  


     A choice of fan diameters at many air-handling capacities will be noted.  The factor of fan revolutions per minute enters here.  For example, a 36" fan running at about 425 RPM delivers the same amount of air as a 42" fan running at only 270 RPM.  Naturally, the slower speed fan will be quieter.

     This rough approximation should be followed by consideration of allowances for local climate and topography.

How Attic Fan Capacity is Rated

     Here, a word of warning should be injected.  The most elaborately designed and carefully installed attic fan may prove to be of little value if its expected performance has been based on unreliable figures as to its capacity; that is, the amount of air which it will move each minute.  All reputable manufacturers of attic fans have their blades tested and rated under exacting conditions.  It is the standard method used throughout the air impeller industry for testing fan blades which are operating against various amounts of static pressure or resistance to air flow.  Since attic fans must draw through windows, grills and other obstructions to air flow, they are operating against a small, but definite amount of pressure and therefore must be rated according to the provisions of the standard test.

Where To Locate The Attic Fan

     Having determined the size of the unit which will be needed, the next problem is to find a good location for installation of the unit, for the ceiling opening which allows the fan unit to draw air from the house into the attic, and for the exhaust area to the outdoors.  The architecture of the house will help decide whether a horizontal or vertical discharge unit is to be installed.  The location of each of the three elements of an installation is determined with an eye first to thoroughness of ventilation, then ease and cheapness of installation.

     In houses of two or more stories, if there is not already a stairway or hatch between the living quarters and the attic, the ceiling opening is usually located in the ceiling of the top floor hallway, allowing air to be drawn up the stairs from the lower floors.  In this way any room may be cooled by opening its windows and a door leading to the stairs.

     Houses which are built on one floor may have the opening in any convenient central location.

     In either type of house a number of smaller openings in different locations may be used if desired, rather than one large central ceiling grill or shutter, providing the installation does not call for a vertical discharge fan or a suction box type of construction.

    Once you have picked the fan, your salesperson will help you pick out the appropriate size shutter or register.

Go to Attic Fans Part 2

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