Power Fans | |
For the most part, a power fan is a motor-driven version of a turbine vent. Like a turbine vent, a power fan uses the rotary motion of blades to negatively pressurize an attic, thus causing hot air to be drawn out. But instead of using wind power to drive the blades, power fans use electricity to drive high-efficiency motors. Unlike a turbine vent, however, the effectiveness of a power fan isn't dependent on wind force. Instead, a power fan is turned on and off as needed, automatically, with thermostat and humidistat controls. (In some models, an integral humidistat control is standard; in most models, however, a humidistat is an add-on option.) Depending on the size of the motor and the efficiency of the blade design, power fans can move more than 1,500 cubic feet of air per minute. That high volume of air movement is critical. To ensure adequate ventilation, power fans must provide at least 10 changes of attic air every hour. (Some models offer a two-speed option that allows fan speed - and air movement - to be determined by the "demand" for increased ventilation.) Although a power fan can move a large volume of air, a single unit cannot "vacuum" all hot air from an attic. Usually, to provide uniform air movement along the underside of roof sheathing, a series of power fans must be spaced equally along a roof. (See illustration) When evaluating the feasibility of using power fans, it's important to evaluate one factor which is considered to be a major disadvantage: namely, that power fans cannot vent away moisture during the winter unless they are equipped with humidistat controls. If this is a problem in your climate, it can be solved, of course. When that's done, power fans do offer key benefits. For one, they ensure a high volume of air flow, even on days when outside air is virtually still (a common occurrence in inland areas on hot summer days). In addition, power fans provide ventilation in some circumstances where fixed systems would prove inadequate. One common example involves hard-to-vent hipped roofs. Most fixed systems fail to meet ventilation code requirements for high vents, while power fans can provide the air needed to ventilate the attic properly. Another example is a tile roof. A fixed system is simply not feasible, while gable-mounted power fans will do the job nicely (although they must be mounted high,on the gable to exhaust air correctly). Source: Air Vent Inc. A Certainteed Company Reprinted with permission |