The product sold here is a 20 watt panel unit manufactured in Phoenix, Arizona by Natural Light Systems ([...]).Unless you have a small attic space, I recommend that you buy this version over the 10 watt model.
BACKGROUND
I have a house with a moderate pitch (I believe about 15 degrees or so), with a black asphalt shingle roof, located in Mountain View, California. When re-roofing the house a few years ago, the contractor recommended installing several small "eyebrow" vents opposite each other along the ridges. They take advantage of the Venturi effect to exhaust hot air. They really weren't sufficient, though. Wind turbine ventilators were too small to retrofit and obtrusive, and over the years I've 1) installed (and removed) a gable fan (noisy and inefficient), and 2) finally re-engineered a large Vornado fan to direct exhause out of a leeward vent. But it was still a bit noisy and used electricity, of course.
Hence, the search for a low-profile solar attic fan, though, I did consider a new passive system from Aura ([...]). That company also has a solar panel version, though there are some features I don't particularly care for. I installed the solar attic fan on the leeward side of the house, facing due south, in place of an eyebrow vent, in the center of the house. I sealed up the opposite windward eyebrow. There are sufficient air intake vents (or soffits). Total attic volume is somewhere between 1,500 and 2,500 cubic feet.
PROS
Simple and straightforward engineering. This unit comes with a single well-designed fan. One brand that I nearly bought, from Aura, is cluttered with several little CPU fans tucked inside the unit.
One piece heavy gauge stamped aluminum body with no seams. The seamless body is leak-proof with no additional caulking needed.
Aluminum and stainless steel hardware, so corrosion shouldn't be a problem.
High quality BP solar panel (the 20 watt panel appears to be the BP-SX320). I understand that most panels degrade in performance over time, so think it's better to start out with a more powerful panel. The solar panel is adjustable - great if you can easily get on the roof and want to pick up the rays from the low spring or fall sun. Out of the box, the solar panel rests flat against the top of the unit; the picture shows it in a raised position.
High quality fan motor and blade; easily replaceable if needed. Extremely quiet - I can't detect any noise from the unit inside of the house, and just a slight whooshing a couple of feet away outside on the roof.
Twenty-five year warranty, thought it's only as good as the company stays in business. I called customer support and found them friendly and knowledgeable. I think the only parts that could potentially need replacing are the fan motor and panel.
Low profile and neutral color helps it blend with many roofing materials and not stick out like a sore thumb.
CONS
Installation is, like, all such roof hardware, a little involved for an average homeowner like me. Just take it easy and, as they say, "measure twice and cut once."
The company provides a silicone sealant, which I regret having used since there are stainless screws holding the unit down. Probably needed for regions where winds get fierce, but I don't think for my area. The silicone gummed up the wires for the thermostat, though didn't do any harm (fortunately, I could clean them off after installation because I have access to the attic).
The solar panel is a bit tough to adjust. I would have appreciated wing nuts rather than Philips screw heads that could easily get stripped.
CONSIDERATIONS
I strongly recommend getting the thermostat, which is a [...] optional item.
Mount on a southern exposure, where there is sun all day (or at least from 10 am or 11 am onwards). In shade, the fan, which is continuously variable speed, will not operate. I believe it will operate, though at a low speed, on a cloudy day. Essentially, the more sunlight, the faster the fan will rotate, hence exhaust more attic air.
You will need to cut a 19" or 16" round hole in the roof (depending on your rafter spacing). 19" is actually fairly large (and what I cut) and will vent a lot of attic air.
Solar panels get dusty which can affect their performance; and birds do perch on them, so direct a little garden hose spray up to them occasionally if it doesn't rain a lot in your area.
DOES IT REALLY DO THE JOB?
I thought I'd conduct a simple test to find out how the unit performed in "passive" state (sans fan) and "active" state (with fan operating).
The set-up: I had one remote digital thermometer in the attic about 8-10 feet away from any vents and midway between the floor and ceiling of the attic, about 20 inches off the floor of the attic. Another thermostat was placed in a shady spot on the patio away from the house.
The test: I chose two comparable sunny, mild days (75 to 76 degrees F. on May 27 and 28, 2009) with a breeze out of the northwest at about 5 mph. On the first day, I covered up the solar panel, so the vent became passive. On the second, the panel was uncovered and the vent was active. I took measurements roughly every hour from noon to 6 pm. I found the patio temperatures peaked on both days at around 76 degrees from 3 pm to 4:30 pm.
The results: During the hottest parts of the day, the attic temperature was 39 degrees greater than the patio temperature when the vent was in passive mode. In active mode, it was 33 degrees greater. The fan, therefore, kept the attic up to 6 degrees cooler. On the first day, I removed the cardboard cover on the panel, allowing the fan to operate, at 4:45 pm (the inside of the house was getting a bit uncomfortable). Within 30 minutes, the attic temperature had fallen 5 degrees.On a day in the 90's, the 30 degree differential was still maintained.
Subjectively, the house seems cooler with the solar attic fan than even with the Vornado - and quieter. If you have a lighter color roof than mine, the reflective properties should reduce the temperature differential. Note that every roof and attic is unique in some ways - with different gable profiles, intake and exhaust vent arrangements and so on. I found these websites helpful: [...] )
CONCLUSIONS
Some experts think that solar attic fans are a waste of money because they're under-powered with unproven life-expectancy. I was hesitant to buy one, but felt this brand offered solid engineering and construction, an unobtrusive low-profile, a powerful enough solar panel, and a great warranty. After two months of living with the fan, I am completely satisfied and unhesitatingly recommend it.
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Solar Attic Fan 20-watt with 25-year warrantyProduct Description: Natural Light 20-watt Solar Attic Fan Features: The environmentally sensible solution that can protect your home and save you money! EASY INSTALLATION - NO WIRING ? Reduces cooling load on AC units? Operates on free solar energy? Adjustable solar panel? Three models available to fit all roof types? Powder coated aircraft grade aluminum? Best warranty in the industry25-year warranty on solar panel25-year warranty on housing25-year warranty on motorWarranty from Natural Light.SOLAR ATTIC FAN WITH 20 WATT MODULE. INCREASES CFM TO 1200. Venting capacity of 1800 square feet of attic space per fan. The 20 watt solar panel is 16.5" x 19.75"This solar unit may qualify for the 30% federal solar tax credit.Begin saving electricity today!
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