Sunday, January 27, 2008
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Friday, September 28, 2007
During the winter months (and in summer when air conditioning is used), an attic exhaust fan represents a potential energy loss because it is essentially a large, uninsulated hole in the ceiling. Since standard fan louvers do not insulate or seal tightly, a cover should be constructed to airseal and insulate this hole. The cover may be installed from the attic side (if attic access is easily available) or from the house side. Covers for both sides 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 in operation.
In addition to sizing a whole house fan correctly, it is important to ensure 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.
A two-speed fan permits the entire house to be ventilated quickly on high speed (such as when the occupants first arrive at home) and then provides gentle air circulation at the lower, quieter speed. A variable speed unit offers more flexibility in selecting the desired air movement.
Control may be a simple on/off pull or wall switch, a multi-speed rotary wall switch, or a timer that automatically shuts off the fan at pre-selected time intervals.
Dampers or louvers typically operate automatically whenever the fan is on. Motorized dampers are available but are not necessary if the louvers are correctly installed and maintained.
MOTOR MOUNTS AND NOISE
A direct drive unit has its fan blades attached directly to the motor shaft. This type of unit is usually less expensive to buy and operates at a higher speed than its belt-driven counterpart. A belt-driven unit typically features a motor driving a slower-moving,larger-diameter fan with four or more blades. It may be quieter, but will require maintenance of the pulley and belt.
A large 18,000 Btu/h window unit air conditioner with an energy efficiency ratio (EER) of 8.8 costs more than 17¢ to operate for one hour. 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 5¢ per hour of use.
In summer, the air inside a home is heated during the hot part of the day. During the
morning, late evening, and night, the outside air is often cooler and can be used to replace the inside air. Operating the attic exhaust fan at these times will cool interior materials. As daytime temperatures rise, the attic exhaust fan can be turned off. The cool materials (along with ceiling or circulating fans, which create an additional cooling effect) will help keep the interior more comfortable.