There is a new product out on the market, well sorta new. It's the solar powered attic fan that sits on your roof and draws air out of your attic for free! Well, $600 free. This is a product who's time has come...to be left at the store where it belongs.
Many of us have the turbine fans that passively turn when we get a little breeze. They are cheap to buy and also free to run. There are also powered fans that have thermostats to turn them on and off depending on attic temperature. They require electricity to be run to them (and paid for). There are also ridge vents that run the entire length of the roof top and vent the attic passivly through convection.
Now the solar fan adds a fourth option. If you have read my previous post you may spot the problem with these fans. True they go in without an electrician and when the sun is shining and you are gaining heat in the attic, they work well. But what happens when the sun isn't shining?
First, as the sun is setting they begin to slow down because of the reduced light hitting the collector. The heat in the attic, from all the radiant heating of the stuff continues to build. As evening comes and the outdoor air cools, the fan stops, just when you need it to be pulling cool air into the attic and cooling all the stuff. So the insulation becomes saturated with heat as it sits all night baking with no cool air.
The next day, the process starts over again except the attic never cooled, so today's heat is added to what's left of yesterday's. The fan runs all day keeping up with the new heat, then shuts off again.
Bottom line - The attic needs venting 24 x 7 in the summer. Spending $600 apiece to vent only when the sun is shining on them, is very questionable.
Maybe there is some justification to adding one of these to an existing complete attic venting solution, I dont know. And for $600 plus installation, I doubt I will find out.
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Wait, why is stuff in the attic hot the first day when the fan is running? If the fan keeps it cool, it's all cool when the sun sets.
ReplyDeleteNot that I'm suggesting paying $600 for one of these.
It was 97 degrees today, mostly sunny.
ReplyDeleteAt 7:00 pm the air is 89 and my attic surface temps are 105. My solar fan would not be running at this point. That 105 would mostly stay there all night unless vented with cooler air.
It will get to 76 as the projected low tonight. (around 5:00 am). You want that cool air to get everything back down to say 80. Tomorrow morning it will begin to heat up again but that's from 80, not 105.
Now I have a ceder shake roof, lots of eave vents and three turbines. A hotter roof would see high temps and thus a higher temp differential.
Or the short answer is, fans aren't really good at keeping things cool, unless those things sweat. Fans really just vent out the hot air and bring in cooler air.
I'm still a little baffled as to why a fan running during the day would let things get to 105. The objects would be radiating off their heat into the cooler air, no? I certainly feel much more comfortable with a fan than without one. Course, I sweat :-)
ReplyDeleteAir isn't a very good conductor. That's why insulation works. So all your stuff is hot and there is cooler air being brought in, but the heat transfer between stuff and air is slow.
ReplyDeleteHumid air is a little better because the water in the air is a better conductor.
So the attic heats way faster than the air can cool it. Sun beats wind...rock, paper scissors :-)
PS, at 9:00 am my attic is now 84 degrees.
ReplyDeleteWOW! Lets just post what ever we want even if Its not true! Karen Anne and Mark here's the truth. I Installed 2-#1250 SRS solar attic fans ($1400)on my house and have been experimenting with 1 VS 2 units now for 3 years now. My 1st Aug bill at this house was $310,(145 degree attic)no fans. After installing 1-unit dropped to 132 degrees and Aug bill dropped to $250. I was impressed, so I installed a 2nd unit and temp went down to 103 DEGREES on a 100 degree day. WOW! And my $300 electric bill is now running between $140-$199 for the next 2-Aug bills. I installed a wireless temp guage in my attic so i can watch the temp everyday. And the units work even on cloudy days up to 40-50% chance of rain. After 50% they stop but the solar attic fans still vent air when the motor isnt working. And the late evening thing, on a 100 degree day my attic is at 103 degrees after the sun drops the temp drops to 90 degrees within 30-mins, and then the wind and open vents cool it off a little more. And in the morning my temp will be 3-5 degrees colder in the attic than the outside temp, so Its even colder. My neighbors are still in the 290-350 range and I just smile!! Andrew
ReplyDeleteInvested $1400
30% tax credit-$600
saving $40-$160 per month=$500-$600 yr PRICLESS
Andrew,
ReplyDeleteThanks for the comment. The point of the post is, what's the best way to vent an attic. My view is it isnt solar fans for the reasons mentioned.
Now if you had turbines before and saw a big benefit to solar active fans I could see your point. But based on your post you went from a non or poorly ventilated attic to a power vented one. Good job but....I contend your results for the money spent would be much better with passive turbines. Your payback would have been in months not years.