Wednesday, June 06, 2012

Solar Attic fans a waste of Money??

Looking at the description of almost every solar attic fan, they like to say "Up to XXX CFM"
They only produce that rate when the angle of the sun is optimal.   In the morning, and more importantly, in the later afternoon and evening, they ramp down the wattage produced, and so the speed of the fan.   At 8:30pm (when the roof is still more than warm after a warm day), it's not moving much at all.

On top of that, even the max rated "Up To" CFM is lower on most of these fans than is recommended for an average sized attic.
My attic happens to be fairly small.  The typical Solar fan - at its max CFM, is still too small for my attic (I probably need around 800CFM).
A fan like this one is rated that highly:
http://www.ussunlight.com/products/solar-attic-fans/1010tr
(They go out of their way to state the CFM, but another site says it's max is 850)

But given overcast Portland - even on a lot of Summer days (and, like I said, the issue with evenings), I don't have a lot of faith.
Solar companies are pretty abusive about the numbers they quote.  They love to put out maximums, but have no guides to realistic performance expectations.   When you have solar panels spec'd out for a roof, there's all sorts of analyses of angle of incidence, path of the sun seasonally, potential shadow sources.  Cloud cover.  The panel manufacturers don't give you that.  They just say 300W/panel or some unattainable number like that.    It's up to the installers to be informed and inform the customers. 

I think these attic fans might be a particularly egregious case of that because they are most wanted late in the day, and (depending on orientation, location, etc) that might be when they're ramping down.  In NW Portland, the West Hills shade us earlier than the rest of the City by a half hour or more (which sucks, in other ways, but certainly for solar).   The manufacturers say nothing about zero power in the evening.

[Info on CFM capability deleted.  I was wrong.  I cannot find any useful CFM stats for these fans.  Pointed out in the comments.]

ALSO:  Some experts believe attic fans should be weak if used at all.  Stronger fans may pull conditioned air from the home up into the attic.   A moderate fan will remove humidity (the original design function).  Any cooling function will be more limited.   For this purpose a solar fan is possibly adequate.  There isn't enough data I can find to really know.

10 Comments:

Blogger jelly andrews said...

This is very informative. Thanks for sharing this one. Great posting!

10/28/2012 7:40 PM  
Blogger Outdoor Ceiling Light said...

Really nice blog …. Thanks for share with us!
Solar Attic Fan

8/09/2013 5:15 AM  
Blogger Rad said...

Good thoughts. Just had an energy audit and nothing was mentioned about attic fan except that if roof vents weren't adequate. Most newer homes have adequate vents.

9/26/2013 5:13 PM  
Blogger John Bond said...

I don't believe it's a waste of money. You can only save by going solar, and you are required to run your attic fan to keep it clean. It may be a little more expensive, but it's totally worth it.
John Bond | http://bigjuneshop.com/category/solar-powered-attic-fan/

4/18/2014 7:22 AM  
Blogger Thiago daLuz said...

Thanks for sharing your perspective. I've been wondering about solar attic fans as well. Fact is, we don't store much in the attic, and hardly go up there, so I'm thinking we may just get a small one to keep things from overheating. Climate control is overkill. Thiago | http://bigjuneshop.com/category/solar-powered-attic-fan/

4/18/2014 7:25 AM  
Blogger Max Rockbin said...

A Note (Author): I purchased a regular attic fan. Vastly more powerful than a Solar Fan. Even with this powerful attic fan - rated for more air volume than my attic needs - it does not significantly cool the attic during the day.

How can that be? After all, the air outside might be 85, but that's a lot less than the attic - over 100F.

The answer: Hot roof. The roof itself gets very hot and that radiates the heat (infrared), just like an infrared heat lamp. A heat lamp doesn't need to move air to make it hot. Same for a hot roof. As soon as the Sun goes down it starts to cool off. That time right after the sun goes down is when an attic fan can be most effective. You can accelerate the temperature drop to match outdoor levels. But when the sun goes down, where is the solar attic fan?

If your attic has insulated rafters, blocking that heat from entering the attic, maybe you would get some benefit, but that's an unusual situation.

If you look up the history of attic fans, they were not invented to cool off attics. The original idea was to remove excess moisture that could build up. For example if your roof was cold and water was condensing on it, but your attic was warmer from the heat below.

I think no one would question that your first move should be insulating the floor of your attic. If you do that well, you will have a hot attic sometimes, but not much of the heat will get to your living space.

I'm not an expert, and I'm sure there are circumstances where some kind of attic fan would have some benefit. Maybe even a solar one if it turns out they're powerful enough to remove moisture like attic fans were originally designed to do. But I would not expect any significant cooling from one.

4/18/2014 9:06 AM  
Blogger depra parker said...

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5/11/2014 11:54 PM  
Blogger Dr. Tanya Lyn March said...

I'm looking into installing a fan for my attic in Portland. I typically use my house fans at night to bring in fresh cold air and shut the windows at night. The solar fan in the heat of the day is bringing in hot air-very puzzled.

7/16/2014 6:57 AM  
Blogger anjila smith said...

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10/12/2014 10:47 PM  
Blogger Mike Lintro said...

I had no idea, thanks for sharing. There are a number of options when it comes to cooling off the attic. Any of the methods you use that work will benefit you a little. Even if they don't work very well. Especially if you are like me, and keep a lot of stuff in your attic. You need it to not get too hot up there. http://bigjuneshop.com/category/solar-powered-attic-fan/

10/29/2014 2:54 PM  

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