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  1. #1
    Join Date
    Aug 2009
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    154

    Attic ventilation

    My neighbor is having a 2 zone hvac system installed with 1 zone having the equipment and ductwork in the attic. I have been wanting to move my system to the attic but haven't because of all the normal concerns (poor efficiency, less comfort, and shortened life due to extreme temps). My neighbor has 2 gable fans blowing out on opposite ends and 1 roof fan blowing out. He has fully vented soffits. He said his attic never gets extremely hot in the summer here in N.J. Is this possible? Could this make the attic setup work well?

  2. #2
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    In most cases, attic ventilation fans pull conditioned air out of the home, so make the AC work harder.
    Any efficiency gain from the attic being cooler is lost, and then some, by conditioned air being pulled from the home.
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
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    352
    Quote Originally Posted by riderman View Post
    Sorry, I gatta raise the BS flag here. I've read these type of posts and kept my mouth shut. Please give numbers to back up your claim. (I'm not allowed to) I'll keep it simple..an attic with X number of sguare CF that is provided with a powered attic fan that draws 1/2 or even equal that equivalent cfm. How does that attic fan "suck out" conditioned air from the living space?

    This is BS propaganda in IMHO.

    I understand disruption of airflow and possibly making ones attic hotter than usual with an improperly installed powered ventilation system. Most of the "facts" available are from "internet posters"... like you and I. lol

    I cannot go into details how attic insulation does not stop airflow, one should seal all penetrations in the ceiling to attic barrier, etc., to prove my point because I'm not AOP.

    I guess the attic ventilation equipment companies only want our money to sell their product. Much like every equipment suplier in the world.

    Can some AOP member explain natural convection heat removal in the attic vs powered? The real numders/facts, not just "it sucks the AC out of your house because of your attic ventilation fan" type of responce.
    According to Energystar:

    "Attic Fan Ventilation
    Attic fans are intended to cool hot attics by drawing in cooler outside air from attic vents (soffit and gable) and pushing hot air to the outside. However, if your attic has blocked soffit vents and is not well-sealed from the rest of the house, attic fans will suck cool conditioned air up out of the house and into the attic. This will use more energy and make your air conditioner work harder, which will increase your summer utility bill.

    You don't want your unfinished attic cooled by your air conditioner. To prevent this, follow the air sealing and insulation strategies in this guide and make sure the attic is well-ventilated using passive vents and natural air flow."

    http://www.energystar.gov/index.cfm?...ic_ventilation
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  4. #4
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    So moral of the story... attic FAU is fine, just make sure you put a vapor barrier to keep house air from leaking and make sure your vents are open so the fan will just circulate outside air through the attic.

    Also, if you are moving your FAU from a conditioned space to a unconditioned space, you will see more heat load but that is just a trade off. Make sure your unit is sized for the extra load.

    Another thing, make sure your Attic can handle the extra weight...
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    4,183
    Electric fan CAN help IF there is enough soffit venting, otherwise it can pull conditioned air out of the house. My favorite attic vents are the old school wind turbines, they work well and don't require any electrical hookup. Ridge vents are mostly worthless, the filters quickly clog if the vents even work to begin with.

  6. #6
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by 54regcab View Post
    Electric fan CAN help IF there is enough soffit venting, otherwise it can pull conditioned air out of the house.
    Increased soffit venting reduces the amount of air being pulled out of the conditioned space, but doesn't stop it.
    Creating a reasonably air tight barrier between the conditioned space and the attic on an existing home can be quite an undertaking.

    If there are any combustion appliances located in, or drawing their combustion air from the attic, powered attic ventilators should not even be considered, no matter how well sealed the conditioned space is from the attic.

    At least one state's energy code outright bans the installation of powered attic ventilators in residential new construction.

    As far as I'm aware, and I've read a lot of studies on the subject, every building science organization that has done any kind of testing on powered attic ventilation came to negative conclusions about them.
    Even under ideal circumstances they consume more power than they save in cooling costs.

    To quote Joe Lstiburek on the subject of powered attic ventilation: "Don't fricking do that!".


    While there hasn't been a single large scale conclusive study on the subject, the preponderance of the evidence from the dozens of small scale studies, various building science organizations recommendations, and the recommendation of building science professionals is extremely lopsided against the use of PAV's.
    If more government is the answer, then it's a really stupid question.

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    4,183
    I'm not a fan of PAV, but I do think there are times they can be appropriate. When set at relatively high set points (110-120F) they can save more than they consume. Dropping a 140F to 120F helps a LOT. Unfortunately most set them @ 90F and they run WAY too much. Using a fan with a PSC motor helps, however most are shaded pole. High quality fans can last, but most of them are cheap and don't make it 3 summers. Solar fans, they are worthless. They claim to defy physics by getting 500CFM out of a 10 watt motor...

    It's the ridge vent myth that I want to point my guns at, they simply don't work, regardless of how much the roofers and "energy experts" push them. Any time I've seen them actually installed correctly the "filter" clogs up just like on a HVAC unit.
    http://www.youtube.com/watch?v=SyIw2GTiKvQ

    Large gable/dormer vents/"fake" attic fans and wind turbines are the only things that have net free vent area to make a real difference. Most other solutions are too small/restrictive to work properly.

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,316
    That smoke test was interesting to watch. The test conditions can be found at this link:

    http://www.roofingcontractorreview.c...moke-Test.html

    When I saw the post date of the video I was a little skeptical, but the posted conditions seem more appropriate for conducting such a test. Still, it would have been better, IMO, for the outdoor ambient to be higher than 80.

    The smoke did nothing but stratify in that attic. Hard to tell in the video if ridge vents were even there. But if they were, as claimed, and soffit vents were present as well, then they are worthless, as shown by the smoke patterns.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
    Jun 2001
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    Moore, Oklahoma, United States
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    I don't see any incentive for the video to lie, it's not like he's trying to push a competing product. The video agrees with what I've suspected all along. For air to escape a ridge vent it has to go through a "hoghair" type filter than make a 135 degree turn, then flow upside down for a few inches. Ain't gonna happen as the video illustrates.
    http://www.askthebuilder.com/roof-vent/

    Wind turbines on the other hand you can FEEL the air exiting through out of the attic, especially if it's windy outside. Even if there is no wind the hot air passively exiting is enough to actually turn the turbines. I found one from Lamaco, but I'd love to see an independent smoke test done on them.

    Another pet peeve I have is black roofs in southern climates, why would anybody do this?
    Last edited by 54regcab; 07-15-2012 at 10:20 PM.

  10. #10
    Join Date
    Nov 2000
    Location
    Eastern PA
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    To not keep the heat down in attics with forced air ventilation because there may be issues with leakage from inside of the dwelling is the dumbest argument I have heard in our industry.

    If there is not enough venting to prevent an attic fan from creating a negative pressure great enough to overcome a well sealed envelope, then more venting needs to be installed.

    If there are leaks in the building envelope that allows for conditioned air to be drawn from the dwelling because of an attic fan, then the leaks need to be addresssed.

    Attics either need to be kept cooler by ventilation or by sealing them and foam insulating the roof rafters. Attic ducting needs to be very well sealed with a solid vapor barrier on properly installed insulation. I prefer ductboard for attic ducting because when it is properly installed, it forms a sealed air system with a constant vapor barrier.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  11. #11
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    Jan 2009
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    Keokuk, IA
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    One more problem with ridge vents. You have a sloped hot roof. The roof, in common zero wind summer conditions, will have a convection current running towards the peak. So you'll have a mass of air moving towards the peak which could offset much of the air movement trying to escape the attic.

    Info like this makes me feel better all the time about pushing the benefits of unvented attics.

  12. #12
    Join Date
    Nov 2000
    Location
    Eastern PA
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    Quote Originally Posted by motoguy128 View Post
    One more problem with ridge vents. You have a sloped hot roof. The roof, in common zero wind summer conditions, will have a convection current running towards the peak. So you'll have a mass of air moving towards the peak which could offset much of the air movement trying to escape the attic.

    Info like this makes me feel better all the time about pushing the benefits of unvented attics.
    I don't understand why you would not promote attic ventillation.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  13. #13
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    Jun 2001
    Location
    Moore, Oklahoma, United States
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    Unvented attics with spray foam applied to the bottom of the roof decking are the ideal setup. Air handler and ductwork is in conditioned space, access for maintenance isn't impeded by itchy insulation. Unfortunately insulation contractors are talking 5 figures to spray foam a typical 2,000 sq ft house in our area.

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