Solar Attic Fan

* See Update Below *

I live in the heart of sunny Florida.  It gets very hot down here in the Summer.  I've checked into getting a commercially available solar attic fan, but those were several hundred dollars.  Too expensive for what you actually get.

Harbor Freight had their 5 Watt solar panel on sale for $39.99.  That's $10 cheaper than their regular price.  Since I have a Harbor Freight credit card, I get gift cards as part of their cash back program.  I had a $20 gift card, so my actual out of pocket expense was $20.  A 5 Watt panel works out to be 416 mA worth of power in full sunlight.  The formula for Power is P=I (current) x E (voltage).  In this case, 5 Watts = I x 12V, so I = 5/12, or 416.6 mA.

I had several 12 Volt muffin fans.  The largest one is 6" in diameter, but is rated at 500 mA.  The solar panel will spin it, but not up to full speed.  The voltage drops to about 8 Volts.  The next largest fan I had was a 3.5" 12 Volt computer fan.  It was rated at 180 mA, well below the rating of the solar panel.  Consequently, the fan instantly jumped to full speed, moving quite a bit of air.  I attempted to connect two small fans in parallel, but once again the voltage dropped.  I suspect the panel puts out less than the rated 5 Watts.  Nevertheless, it runs one fan nicely.


Before mounting the fan & panel, I tested several configurations.  Once I found the ideal setup, I grabbed my drill, screws, and some silicone RTV.

I mounted the solar panel right next to the gable end vent.  The power cord drops through a hole I drilled in the roof.  I used a liberal amount of silicone RTV on the power cord and screw heads to prevent leaks.

This is the inside of the gable end vent.  Man, is it hot in here!  The ridge vents aren't very effective.

The plate with the fan is simply screwed into place.  It fully covers the gable vent.


The actual installation took 1/2 hour.  I took a couple of days to determine the best mounting location, based on the sun in the morning & evening, shadow from a maple tree, etc.  I had considered mounting it on the patio roof, making it completely hidden, but that area is shaded in the afternoon.  Turns out mounting it next to the vent was the ideal spot.

Once it was finished, I went back on the roof and put my hand near the gable vent.  Felt like a dryer vent.  The fan moves a lot of air for its size, and the air was hot.  Good sign! 

So, for $20, I have a solar attic fan.  It only runs when the sun is out, but that's when it's hottest.  It's quiet, costs nothing to operate.  If I can later find a larger 12 V fan that draws 300 mA or less, I'll give that a try.


Update 1


The original fan worked well for its size, but just didn't move enough air to be effective.  I bought a 12V fan in the camping section of Wal-Mart, for all of $10.  The battery compartment cracked the first time I tried to use it.  Already chucked the receipt.  So rather than trash it, I mounted it in a piece of scrap plywood, and put it in the attic.  The 5W solar panel used to power the small muffin fan was too small for this larger fan.  I used a Harbor Freight 15W solar panel, which can provide a solid 700mA (or more) during the peak of the day.  This fan draws a little under 450mA at 12V.  No regulator, no sensor.  Sun up, fan runs.  Sun down, fan off.  Simple!

$10 at Wallyworld

95+ degrees in my garage.  Over 120 degrees in the attic.

Overcast day, but still enough sunlight to spin the fan.

Jeez it's hot up here!  Very simple install.  Cut a hole in the plywood, drilled & screwed the fan to the plywood, then attached the plywood to the 2x4s around the gable end vent.  Sealed the plywood to the gable end with pieces of foam to prevent the fan from blowing hot air back into the attic.

The panel to the left now powers the fan.  I came up with an easier method to mount these 15W Harbor Freight panels.  Details at this link.


Update 2


The camping fan worked fine, but it had a downside.  It used a basic brushed motor, which produces RF noise across the same frequency as my garage door opener.  Consequently, with the fan running, the remote wouldn't work with the opener.  It took a while to find out why the opener quit working!

I replaced the larger camping fan with two brushless muffin fans.  These are often used in computer power supplies.  I believe I only paid a couple of bucks for each of these fans from a surplus website.  I didn't have the correct size finger guard, so I used some intended for a larger fan.  Works fine.  Take it from someone who sliced the snot out their finger with a muffin fan, please use the guards!

The MorningStar charge controller I have automatically turns on lights at night via the built-in relay.  I added a larger relay to it, allowing me to power up a small inverter, which powers CFL outdoor lighting. When the sun comes up, the controller turns off the relay, which turns off the inverter/CFL lights. By using the other side of the relay, it then powers up the fans. Sun up, fans on, inverter off. Sun down, fans off, inverter on. Simple & automatic.

The black board is a paper/foam product. It was a piece of scrap from a graphics shop. This is used as backboard for posters. Could have used wood, but this stuff cuts/drills easily. Not structural, but fine for this application. Might have $5 invested in this project.

I used a slightly larger brushless muffin fan in the gable vent as well.  I'll post pictures of that one later.  Not using any of the camping fans anymore.  Too bad they're so RF noisy, they moved plenty of air, and are inexpensive. 

The show side.

The business side.  Could have put more fans in the panel, but the noise increases with each fan.  I don't know what the CFM rating is, but they move enough air to blow papers off a workbench!

El-Cheapo fans.  I will periodically peel up this sticker, and apply a drop of oil on the bearing.  I bought 6 of these fans to make the shipping cost worthwhile.  If one fails, it'll be a quick swap.

The panel is attached via screws to the attic access panel.  The panel can be swung up, or even completely removed, for entry into the attic.  The 12V power cord has Anderson type connector for quick disconnect.




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Last updated 05/18/11   All rights reserved.