Monday, May 9, 2011

Solar Powered Attic Fans

Many manufacturers now offer solar powered attic fans to ventilate attics and help keep attics cooler. Solar powered fans rely on a small (typically 10- or 20-watt) solar panel to power a DC motor when the sun is shining. The fans, which exhaust air at a rate of 800 to 1200 cfm, are installed with intake vents (such as soffit and gable vents) to provide high-capacity powered ventilation without electric operating costs. Most vents are mounted high on the roof, near the ridge, and combined with soffit or gable vents for balanced intake and exhaust air streams. Solar powered gable ventilators are also available.

Because they cost nothing to operate, solar attic fans are more affordable to operate than conventional powered attic fans.

By reducing attic temperature, attic fans can help reduce summertime cooling loads while at the same time providing ventilation without added utility load.

Ease of Implementation

In new construction applications, roofers will usually install powered ventilating units. The solar units eliminate the need for an electrician to rough and finish wire the units.
For retrofit projects, a roofer or do-it-yourselfer can install a solar-powered attic fan using conventional materials, tools, and techniques

Initial Cost  
Retail prices range from about $350 to $600 depending on ventilation capacity, manufacturer, and optional features of the unit, such as a thermostat.

Operational Cost  
Solar powered fans are fueled by the sun, so there is no cost of operation.

U.S.Code Acceptance
Section R806.2 of the International Residential Code specifies the amount of ventilation required for attics in newly-constructed homes. The required vent area can be reduced with installation of a ceiling vapor barrier or ventilators located in the upper portions of the space to be ventilated. The code does not require powered ventilation for attics.
There may be difficulty using solar attic fans in Dade County, Florida, and areas where building products must approved by the inspection department. The concern in high wind zones, such as coastal Florida, is that roof penetrations of any sort provide a potential weak point in the building envelope. High winds can blow away protrusions leaving the building susceptible to damage from the rainwater that accompanies the wind.

Units typically come fully assembled and are self-flashing. Installation is straightforward and most manufacturers offer clear installation instructions, often with diagrams and pictures. Powered attic vents are designed to be used in conjunction with sufficient intake air vents, such as soffit or gable vents. Units can typically be supplied and installed by a roofing trade contractor.

Warranties range from five to twenty-five years depending on component and manufacturer.

Compared to powered vent fans, there is no need for electrical wiring, and a solar ventilator uses no electricity (hence avoiding operating cost). Although equipment costs are greater for solar powered attic fans than conventional powered fans (about $200 more), the cost to bring electrical wiring to the attic to supply a conventional ventilator closes the gap on installed cost.
Ventilation is only provided when there is ample sunshine to power the fan motor. The highest solar insolation (and, hence, fan speed) typically coincides with the time of greatest need for attic ventilation.
If there is inadequate attic intake air and poor sealing between the conditioned space of the home and the attic, powered attic fans can potentially draw air from the house into the attic. Not only can this compromise energy efficiency, it can increase the risk of attic moisture problems as well as increase the risk of drawing the byproducts of combustion into the house (a process called backdrafting).

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