A whole house fan is a simple and inexpensive method of cooling a house.
What are the benefits?
- First cost benefit
- Equipment cost for whole house fan = $300 - $800
- Equipment cost for window unit AC = $250 - $750
- Equipment cost for central AC = $2,500 - $5,000
A whole house fan can be used to change the air in the house and vent odors quickly.
- Economics of operation
Operating a properly sized 2.5-ton air conditioner with a seasonal energy efficiency ration (SEER) of 13 in California, costs over $43 per cooling season (224 hours), based on 10.0¢/kWh, or roughly 17.5¢ per hour of runtime. A large 18,000 Btu/h window unit air conditioner with an energy efficiency ratio (EER) of 9.8 costs more than 17¢ to operate for one hour, but only cools one room. By contrast, a whole house fan has a motor in the 1/4 to 1/2 hp range, uses 120 to 600 watts, and costs around 1¢ to 6¢ per hour of use.
What are the drawbacks?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 vs. attic fansA whole house fan is installed in the ceiling of a home and moves air from the living space into the attic. It forces hot air out of the attic and pulls cooler air from the outside of the house into the living space, which is ideal for people who come home to a hot house when it is cooler outside. It costs less than central air conditioning to install and operate and uses already cool air. (Attic fans only exhaust hot air from the attic space not the living space).
Cooling StrategiesIn 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 eighty-five degrees.
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 keep the interior more comfortable.
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.
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 it 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 FanDetermining the amount of airflow in cubic feet per minute (cfm) that the whole house fan should provide involves a simple calculation. Multiply the total gross square footage of the house (include the 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' x 40', 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.
Maximize your SavingsDuring 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.
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 CoverA 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" 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
Label the attic-side box cover
Attach labels to remind users to remove energy-saving covers:
- “Whole House Fan Cover”
- “Remove Before Operating Fan”
- “Replace When Not Using Fan”
Installing a Whole House Fan
Use “H” Brackets to Provide Proper SupportWhen 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.
“H” brackets make fan installation easy. A fan with a 34" base (30" blade) will work with the dimensions shown in the diagram to the left.
Installation Tips and ConcernsSeal 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, (4,5000 / 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 one-half 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 interface.
Care should be taken to avoid backdrafting 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 backdraft a water heater located inside a louvered closet door.
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.
- Whole House Fan [PDF: 236k]
*Special thanks to the U.S. Department of Energy, Building Technologies Program for allowing us to reprint the contents of this article and supplying the pdf for our customers to download.