Solar attic fan

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I want to put a solar attic fan on the roof. I live in central Texas. Today 104 with heat index of 116. I need to cool my attic. I have 4 vaulted ceilings in the house which I can't get to to further insulate. I have what we call "turtle backs" for ventilation. Soffit vents as well. I have only 6 turtle backs for the whole house. House is 2800 sq.ft with 12/12 roof. Tons of attic cubic footage. I haven't measured but will guess 160 or more in the attic. I used a cigarette test for the soffit vents to see if any draw was happening. No smoke into the soffit vents which to me means little ventilation. Question. If I put a solar fan on the roof do I need to cover the turtle vents? Would an attic fan just pull through the turtles and not through the soffit vents? Any help wold be appreciated.
Rick

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Solar attic fan

    Here's a thread about solar attic fans:

    http://forum.solar-electric.com/showthread.php?t=3045

    We've also discussed ventilation in other threads, which aren't titled as such and I can't remember where they are!

    You are right to be concerned about the possibility of the fan drawing from the turtle vents instead of the eaves. Air circulation can be a tricky thing to predict. Generally, you want the air coming in from the eaves and out through the highest point possible. Under ordinary circumstances it should be possible to do this with convection. However, modifications to make that possible frequently require major changes to the roof to add ridge venting. I suggest you try the fan at a gable end, and if you don't detect drafting through the eave vents start blocking up the others from inside - nearest the fan first.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Solar attic fan

    Do you have a link to a website that describes "turtleback vents" for us non-texans?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Solar attic fan
    mike90045 wrote: »
    Do you have a link to a website that describes "turtleback vents" for us non-texans?

    Mike:

    I think he's referring to the standard 'lump' style roof vent, which does look a bit turtle-ish with the right amount of imagination! :D
  • FL SUN
    FL SUN Solar Expert Posts: 94 ✭✭✭✭
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Solar attic fan

    Yes: many variations on the theme. Do a Google image search for "turtle vent" and you'll see.

    I really like that second link, FL SUN! If those could be put in place of existing vents without a lot of roof damage they should be just the answer for the OP.

    And unlike 'turbine' vents, they'd actually work. :p
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
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    Re: Solar attic fan

    Okay, vents and Texas - that reminds me of a story.

    I was born in Texas, and after my parents divorced my mom moved away with myself and my brother, but my Dad stayed in Texas. In 1984 I had to go to Dallas and take care of him as he was dying of Cancer.

    One Sunday morning, I woke up about 6am and could smell that coffee was already made, so I knew my Dad was up. I went into the kitchen and he was sitting in his dining room chair, looking out the sliding glass back door which was open a couple of inches.

    He had his 30-30 lever action (with scope) in his lap.

    I said, "Uh."
    He said, "Hear that?"

    Indeed I did, it was a horrid squealing noise, very much like fingernails on a blackboard, but loud enough that the whole neighborhood could hear it.

    He pointed to a roof turbine on a house about 150 yards away, and said, "I've asked that #$^%$#$% a dozen times to fix that thing or at least oil it, and I know half the neighborhood has mentioned it to him as well. Well, today I'm going to fix the damn thing for him."

    And he did. He sighted his rifle and at 6am on a nice quiet Sunday morning, the whole neighborhood was treated to the sound of a 30-30 going off.

    "Sqqeeeeeeeeeeeeeecccchhhhhhh" "BANG"
    Then silence.

    Later, I looked at the thing with binoculars, and he'd tagged it exactly where the rotating head meets the air tube. I drove around and looked at it from the other side and there was an exit hole that you could put your fist through. Pretty much blew out one whole side of the rotating head.

    Several of his neighbors congratulated him on his fine shooting.

    Yup, Texas is like that sometimes.
  • FL SUN
    FL SUN Solar Expert Posts: 94 ✭✭✭✭
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    Re: Solar attic fan

    dwh - that is hilarious! Not too much a 30-30 can't fix fo sho! Those turbine vents sure do make a squeak that gets next to you no doubt.

    Cariboocoot - that is a great idea to replace those turtlebacks with the solar fans. I have that particular fan, and it moves a surprising amount of air, although not as much as a 120 volt powered one. It will take several more fans to provide the same amount of air.

    I do believe these solar vent fans are inclusive of the 30% tax credit offered in the U.S. If so, they are a much better deal in the long run than those powered by line voltage.

    Home Depot carries these for $217.00 each in my part of the world.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar attic fan

    The link below does show what we call "turtle backs". I found a Natural Light 20 watt solar fan for $407.00 says it will move 1200 cfm. I think I will start with one and add one or two more in the future. I'll check when installed to see if I am getting more ventilation from the sofit vents. If not I will start covering the turtles one by one. I would like ridge vents but my roof line won't work plus I don't have any gable ends to put a gable fan.
    Rick
  • BB.
    BB. Super Moderators, Administrators Posts: 33,474 admin
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    Re: Solar attic fan

    If you have 120 VAC power available in your attic--you might be better off with a thermostatically controlled standard AC attic fan of some sort.

    Generally they move a bunch more air than the solar fans (10x or so?) and the electricity used is not much (perhaps a couple cents per hour when the fan is running).

    Add all of that together--and solar fans don't usually make good economic sense.

    Can you (architecturally) add a copula to your roof? The added height and venting may be a good (no power or powered) alternative).

    Make sure your sofit/low vents are clear (not covered by insulation) and open... And your high vents (in sq. inches) should be equal or larger than your low/sofit venting (air expands as it is heated).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Solar attic fan

    dwh - Your Dad may have found the only good use for those 'turbine' things! :p
  • JetMech
    JetMech Solar Expert Posts: 71 ✭✭✭✭
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    Re: Solar attic fan

    The gable fan option got me thinking...I have a whole house fan that exits through our three gable vents (and possibly the ridge vent as well) in our attic. Is there any problem mounting a solar powered gable fan that may be "driven" by the air exiting the attic when the whole house fan is on?
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
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    Re: Solar attic fan
    JetMech wrote: »
    The gable fan option got me thinking...I have a whole house fan that exits through our three gable vents (and possibly the ridge vent as well) in our attic. Is there any problem mounting a solar powered gable fan that may be "driven" by the air exiting the attic when the whole house fan is on?

    Nah, you won't hurt it that way. Those fans will turn when there is voltage from the solar panel, but they'll freewheel when there isn't. Go for it.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar attic fan

    Can't add a cupola to the roof. I have looked at powered fans. Questions I have are don't they use $30-40 electricity per month? The whole idea is not to add to my electric bill. It's high enough already.
    Rick
  • BB.
    BB. Super Moderators, Administrators Posts: 33,474 admin
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    Re: Solar attic fan

    Put in your own numbers (size of your fan, price per kWhrs, how long it will run)--but assuming 300Watt / 0.3 KW motor, $0.10 per kWhr (for Texas), and 6 hours per day (thermostatically controlled--I am guessing at 6 hours per day--it may be 12+ hours per day for your home :confused: , I just do not know):

    0.3 kWatts * $0.10 per kWhr * 6 hours per day * 30 days per month = $5.40 per month

    Remember that this is a ~300 watt motor--and most solar attic fans are 10-20 Watt motors--so a well designed AC attic fan will move ~10x as much air as a typical solar attic fan.

    If I have AC power available--I probably would want 1x AC Attic Fan at $6 per month + 1 installation + the cost of one AC fan; vs 10x solar attic fans + 10x installation + 10x any maintenance issues.

    Yes, you are adding yet another load to your already high electrical budget--but if you can save 9x $300-$600 in DC attic fans (plus installation)--that will pay for a lot of AC power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
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    Re: Solar attic fan
    rkted wrote: »
    Can't add a cupola to the roof. I have looked at powered fans. Questions I have are don't they use $30-40 electricity per month? The whole idea is not to add to my electric bill. It's high enough already.
    Rick

    Okay, don't think I've mentioned this before...

    Once, I installed one of the solar powered gable fans (the Home Depot kit for $200), but not in a gable...

    The customer had an existing 110v attic fan, which had burned out its motor. Something like this:

    http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100072633

    From underneath, there was an MC running into a thermostat box (which you can see in the Home Depot pic) which was connected to the motor.

    I yanked out the 110v motor and replaced it with the "solar gable end kit" motor - then ran it off the solar panel.

    Here is the solar kit:

    http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100548465&N=10000003+90048+502892

    Note how the motor is held to the circular housing by three mounting arms. There is a circular clamp around the motor body, with these arms sticking out, and the arms have a bend at the end so they can be bolted to the circular housing.

    This is exactly the way the 110v motor was mounted in the original ventilator.

    And - the three arm mount for the solar motor was a perfect fit for the existing vent housing.

    The only problem was that the existing vent had the arms spot welded to the circular housing, so I had to cut those off and drill three holes to mount the new solar powered motor.

    Bing, bang, boom. Done deal.

    You could probably mount the fan under your "turtlebacks" (whatever those are) if they'll flow enough air with the fan running.

    You don't *have* to mount the thing in a gable end you know. :)


    EDIT: I think I just realized what turtlebacks are...some type of "roof louver". Anyway, if there is room and they are big enough, you might still be able to mount the fan underneath and seal it with some metal duct tape or some such.