Cooling - Installing And Using A Whole House Fan - 5/1/2004 - House Systems HVAC Heating Air Conditioning

Cooling - Installing And Using A Whole House Fan

Why Use A Whole House Fan? A whole house fan is a simple and inexpensive method of cooling a house. The fan draws cool outdoor air inside through open windows and exhausts hot room air through the attic to the outside. The result is excellent ventilation, lower indoor temperatures, and improved evaporative cooling. 

What Are The Benefits? A whole house fan can be used to change the air in the house and vent odors quickly. A whole house fan can be used as the sole means of cooling or to reduce the need for air-conditioning. Outside air temperature and humidity dictate times when the whole house fan would be favorable over air-conditioning. If both methods of cooling are present, a seasonal use of the whole house fan (during spring and fall) may yield the optimum combination of comfort and cost. 

Cost benefits:
* Equipment cost for whole house fan = $150-$350
* Equipment cost for window unit AC = $250-$750
* Equipment cost for central AC = $2,000-$4,000

What Are The Drawbacks Of Whole-House Fans? Temperature, humidity, and dust. A whole house fan has some drawbacks - the fan can only cool the inside of a house to the outside temperature; unlike an air-conditioner, it does not dehumidify; and dust and pollen can be brought into the house. Whole-house fans can be noisy, especially if improperly installed. In general, a large-capacity fan running at low speed makes less noise than a small fan operating at high speed. All whole-house fans should be installed with rubber or felt gaskets to dampen noise. You can set a multispeed fan to a lower speed when noise is a problem. 

A whole-house fan can substitute for an air-conditioner most of the year in most climates. Whole-house fans combined with ceiling fans and portable fans provide acceptable summer comfort for many families, even in hot weather. The whole-house fan pulls air in from open windows and exhausts it through the attic and roof. It provides good attic ventilation in addition to whole-house ventilation. You can regulate cooling by simply closing windows in the unoccupied areas and opening windows wide in occupied areas. Many people cool the bedrooms at night and the living areas during the daytime. Whole-house fans should provide houses with 30 to 60 air changes per hour (varies with climate, floor plan, etc.-check with a professional to determine what is appropriate for your home). The air-change rate you will choose depends on your climate and how much you'll depend on the whole-house fan for cooling. Cooler, shadier areas don't require as much ventilation as warmer, sunnier ones. Houses entirely dependent on whole-house fans require a bigger fan because there is no air-conditioning to fall back on. 

Cooling Strategies: In the summertime, the air inside a home is heated during the hot part of the day. At night especially, and during the morning and late evening, the outside air is often cooler and can be used to replace the inside air. It is important to open all or at least several windows, even if only partially, to provide adequate airflow. Closing windows in unused rooms will create higher velocity air movement in occupied rooms. Running the whole house fan whenever outdoor temperatures are lower than indoor will cool the house. Operate the whole house fan throughout the evening to cool interior materials. An approximate rule of thumb would be to use the whole house fan when outside temperatures are below 85 degrees F. As daytime temperatures rise, turn off the whole house fan. The cool room materials (along with ceiling or circulating fans which create an additional cooling effect) will help keep the interior more comfortable. 

During the winter months (and summer when air-conditioning is used), a whole house fan represents a potential energy loss because it is essentially a large, uninsulated hole in the ceiling. Standard fan louvers do not insulate or seal tightly. Some fans come with a tight-sealing winter cover (or you can build one). If you switch between air-conditioning and cooling with a whole-house fan as the summer weather changes, build a tightly sealed, hinged door for the fan opening that is easy to open and close when switching cooling methods. 

Build and use fan covers: Because the louvers are leaky, a cover should be constructed to air seal and insulate this hole during the seasons when the fan is not in operation. The cover may be installed from the attic side if attic access is easily available or from the house side. Both covers could be included in excessively hot or cold climates. Homeowners must remember to remove cover(s) before operating the fan and to replace cover(s) during seasons when the fan is not in use. 

How To Build An Attic-side Box Cover: A typical whole house fan has a 30" diameter blade with a sheet metal cowling of 31" to allow for blade clearance. An attic-side box cover may be constructed from a 4' x 4' piece of 1" rigid fiberglass duct board. The box will be 33" square with 1" thick walls (inside dimension of 31" x 31"). It will be 6 1/2" deep. Adjust dimensions to actual fan size.

Attic-side box cover materials list: 
* 48" x 48" piece of 1" fiberglass duct board
* Silver duct tape or house wrap tape
* Tools; measuring tape, straight edge, utility knife
* Permanent marker to label box
* Wear gloves and eye protection when working with duct board 

Use "H" Brackets To Provide Proper Support: When installing a whole house fan, be sure to provide proper support and seal the unit into the rough opening in the ceiling. Never cut a truss chord; wooden "H" brackets installed between the trusses create a framed box to raise the fan above the truss system. The louvers must be able to operate freely (open/close) and care must be undertaken to prevent binding or misalignment.

Helpful Reminders: Attach labels to remind users to remove energy-saving covers. Label the attic-side box cover:
* "WHOLE HOUSE FAN COVER"
* "REMOVE BEFORE OPERATING FAN"
* "REPLACE WHEN NOT USING FAN" 

Selecting A Whole House Fan:
* Fan speed: Two-speed fans permit the entire house to be ventilated quickly on high speed (such as when the occupants first arrive at home) and then provide gentle air circulation at the lower, quieter speed. Variable speed units offer more flexibility in selecting the desired air movement. 
* Control options: Controls may be simple on/off pull or wall switches, multi-speed rotary wall switches, or a timer which automatically shuts off the fan at pre-selected time intervals. 
* Louvers: Dampers or louvers typically operate automatically whenever the fan operates. Motorized dampers are available but are not necessary if the louvers are correctly installed and maintained. Proper opening and closing of louvers is critical to a whole house fan's performance. 
* Motor mounts and noise: A direct drive unit has its fan blades attached directly to the motor's shaft. It is usually less expensive to buy and operates at higher rpm's than its belt driven counterpart. A belt driven unit, which typically features a motor driving a slower moving, larger diameter fan with four or more blades, may be quieter, but will require maintenance of the pulley and belt.

In addition to sizing a whole house fan correctly, it is important that ALL penetrations between the attic and living space are sealed and that the attic is properly ventilated. A central hallway, or a stairway in a two-story house, is the most common location. 

Sizing A Whole House Fan: Whole-house fans are sized in cubic feet per minute (cfm) of ventilating power. To determine the size you'll need, first calculate the volume of your house in cubic feet. To do that, multiply the square footage of the floor area you want to cool by the height from floor to ceiling. Take that volume and multiply by 30 to 60 air changes per hour (depending on the power you need). Then, divide by 60 minutes to get the cubic feet per minute of capacity your house requires.
[(Square feet ______  x  room height ______) x 30 or 60/60 = cfm required _________. ].

Another method you can use to size the fan: Multiply the total gross square footage of the house (include upstairs area) by the ceiling height (typically 8 feet). Select a fan that delivers between one half to one times that amount of cfm at 0.1" static pressure. For example, a 25'x40', one-story home is 1,000 square feet and would need an 8 x 1,000 x 0.5 = 4,000 cfm fan or better. A manufacturer sells a two-speed unit that delivers 4,500 cfm at the high setting (240 watts) and 3,200 cfm at low (120 watts); this unit should be adequate. 

Installing a Whole-House Fan: Installing a whole-house fan is tricky and should be done by a professional. Attic measurements, dedicated circuit wiring, and possibly new attic vent installation should be performed by an experienced person. Attic ventilation must be increased to exhaust the fan's air outdoors. You'll need 2 to 4 times the normal area of attic vents, or about one square foot of net free area for every 750 cfm of fan capacity. The net free area of a vent takes into account the resistance offered by its louvers and insect screens. More vent area is better for optimal whole-house fan performance. 

Installation Tips And Concerns:
* Seal penetrations and vent attic adequately: Caulk all penetrations between the attic and living space, i.e., electrical boxes for ceiling light fixtures, loose attic hatches, large cutouts for plumbing vents, exposed beams, and recessed lights. A whole house fan creates a positive pressure in your attic and it is important that air from the attic is not forced back into the living space through cracks and gaps.
* Guidelines for sufficient attic vent area is one square foot of net free vent area per 750 cfm of fan airflow, (4,500 / 750 = 6 square feet for the example above). Continuous ridge and soffit vents are usually more than adequate. Vents with insect screens may have a net free area equivalent to 1/2 of the total open area depending upon the size of the holes in the screen area. Insulation should be installed directly against the fan box frame. Blown-in insulation may require the sides of the fan box to be raised (with baffles) to prevent interference. 
* Avoid back drafts: Care should be taken to avoid back drafting combustion appliances that are installed in the conditioned space. It is strongly recommended that combustion appliances NOT be installed in such a manner that they use room air for combustion. The whole house fan is capable of pulling large quantities of air from the home and, particularly if not enough windows are open, may easily back draft a water heater located inside a louvered closet door. This could lead to lethal carbon monoxide entering the livable spaces of the house!
* Label your switches: Controls should be placed higher on walls than light switches to avoid confusion and to keep them out of the reach of small children. Labels over switches are recommended to remind users to remove any energy-saving covers and to open at least two or more windows before using.