Keep me cool!
June 14, 2010 12:51 PM   Subscribe

Solar powered attic fan + powered attic fan = cool house or armageddon?

Long question short: Is it possible to hook solar + electric attic fans together so that the electric fan kicks on automatically when the solar is dormant, and then turn off when the solar comes back on?

I'd really like to keep the solar powered fan for obvious reasons (trying to cut back my ecological footprint) and I have a heat pump that does a great job for a/c, but again, would like to cut down on my usage of it.

I live in the Seattle area and recently installed a solar powered attic fan. On our first "hot" day it worked wonderfully! Kept my house at a nice, even 71 degrees. However, as soon as the sun started to set, the temp in my house climbed 6-7 degrees over a couple of hours.

I considered adding a second panel so it could catch the late afternoon rays, but that wouldn't help once the sun went down completely.

Clearly I didn't think through the solar powered issue.

So I started to think about adding a backup electric-powered fan, but I obviously don't want them both running at the same time for fear of negative pressure (or whatever it's called).

I thought a timer would be helpful, say have it come on at 530 every day--but that wouldn't help on the days when it's quite warm but cloudy.

Possibly adding a thermostat to the electric so it comes on 10 degrees higher than the solar? I assume there would still be a risk that they'd end up both running at the same time.

Any help or suggestions would be most appreciated. Thanks!
posted by Zoyashka to Home & Garden (10 answers total) 3 users marked this as a favorite
 
Best answer: Well, if you're the handy sort, you could fairly easily wire a relay into the solar powered fan that would actuate your electric fan whenever the solar fan turned off.
You'd need a controller such as this one - I can't immediately tell if that unit will operate as a "normally closed" unit, which is what you need - relay is closed (i.e. circuit is on) until voltage is applied (your fan is running) which would then turn off the electric fan.
posted by davey_darling at 1:08 PM on June 14, 2010


Response by poster: Thanks davey_darling....I'm definitely not handy, but my buddy is a professional handyman and I'm sure this would be within his abilities.

I'll do some googling for a "normally closed" controller!
posted by Zoyashka at 1:41 PM on June 14, 2010


Response by poster: I've got soffit vents along that take care of the intake for the solar fan--I just worry about have too much suction power between two vents (approx 800 sq ft of attic space).
posted by Zoyashka at 1:44 PM on June 14, 2010


What about a whole house fan in addition to your solar attic fan? They are more effective in cooling your house than an attic fan (which solely vents your attic and doesn't move air in the living areas of your house) and run for a few pennies an hour. It works by drawing air in from the outside through open windows and vents up and out your attic vents. I can't imagine living without mine.
posted by cecic at 1:44 PM on June 14, 2010


Best answer: Seems like a waste to have 2 different fans... rewire the original fan.

What you should have is the one fan which is solar powered with battery backup , or electric grid backup based.

Then use the controller that davey_darling suggested to control what powers the original solar powered fan.

You could set the controller to run the fan based on temperature and/or time

And when the solar power isn't kicking in, back up on the electric grid, or battery pack depending on method. (I would suggest to backup on grid power 1 because it would be cheaper than buying batteries, and 2 the solar cell probably isn't big enough to charge the battery bank as well run the original fan)
posted by MechEng at 1:55 PM on June 14, 2010


Response by poster: Interesting MechEng...do you know of any information on the webs that might show us how to go about doing that? I agree about 2 fans being a waste, I just didn't think there was another option.

I had considered the batteries but decided against it (for the whole "clouds = no power" problem).
posted by Zoyashka at 2:03 PM on June 14, 2010


On our first "hot" day it worked wonderfully! Kept my house at a nice, even 71 degrees. However, as soon as the sun started to set, the temp in my house climbed 6-7 degrees over a couple of hours.

I suspect the lag in temperature was due to it being your first hot day. It took all day for your previously relatively cool attic to heat up to the point where it swung the rest of your house up in temperature. The fact that the sun had gone down at the point is only incidental.

Why do I suspect that? Because your fan has a 10 watt solar panel. The linked page didn't give a cfm (cubic feet per minute) rating for the fan but 10 watts sounded way, way small to me, to the point where it's probably almost entirely ineffective. So I looked up a conventional hardwired fan in a similar style to your solar fan and found this fan. This fan is rated for 1080 cfm and up to a 1600 square foot attic. It uses 2.6A at 115V.

That's 300 watts. The best your panel will provide is 10 watts (sun at an optimal angle to the panel around noon during summer on a clear day) so you know your fan is moving much, much less air at the best of times. If possible, I'd return the fan and install a conventional fan.
posted by 6550 at 2:35 PM on June 14, 2010


Okay, almost entirely ineffective was an overstatement! I did some more searching and turned up some mixed results on solar fans. Some people find them relatively effective but others seemed to need multiple fans to move enough air.

I also looked at more solar fans and I'm skeptical about the claims of 800-1000 cfm on 10-20W of power. How does a solar fan move 800 cfm on 10W when it takes a conventional fan 300 W to move 1000 cfm? Are the mains AC motors and fans so vastly inefficient compared to the solar DC models? The solar fans use 3% of the power to do a similar job? That seems magic.
posted by 6550 at 4:24 PM on June 14, 2010


Here's what I would do. Go to a golf cart store. Buy a battery charger for an electric golf cart. Get three 12 volt batteries. Hook up the fan to the batteries in series, giving you a 36 volt power supply. Hook up the solar panel to the batteries. The solar panels will charge the batteries when the sun is out. The golf cart charger will charge the batteries only when they need it. The fan will run whenever the temperature gets too high in the attic. It won't be quite as efficient because it is operating at 36 volts instead of 38 volts, but it will be, on average, better than what you have now. You might want to interrupt the circuit between the charger and the solar panel so it doesn't burn up the panel. All of this technology is understood by any RV dealership, Radio Shack store manager and golf course superintendent. Try those three sources in your home town for particulars.
posted by Old Geezer at 5:17 PM on June 14, 2010


I also looked at more solar fans and I'm skeptical about the claims of 800-1000 cfm on 10-20W of power. How does a solar fan move 800 cfm on 10W when it takes a conventional fan 300 W to move 1000 cfm? Are the mains AC motors and fans so vastly inefficient compared to the solar DC models? The solar fans use 3% of the power to do a similar job? That seems magic. -6550

With 10W, you can move 800cfm at 0.1" of water (pressure across fan). I'm trying to remember if that's realistic (ie will it do that in the configuration shown, with a 14" fan)... and it just may. Your 300W fan at 1000CFM will be able to move the air with a much higher differential pressure.
posted by defcom1 at 8:44 PM on June 14, 2010


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