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  1. #14
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    Apr 2002
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    call it tempered if you like. The purpose of the tempering is to keep it from getting stagnant, making sure it does not get too humid. Building Science site has scenarios they are blowing 25 CFM and calling it conditioned. If you have gable walls you need to insulate them also,].

    I will look, I have some numbers for when I was keeping my place at 75, off the top of my head the attic did not follow suit exactly,it did not average three degrees lower.

    SO it would sort of being like heating your house to 72 in the winter and it is only 68 in the basement

    Malls with barjoist, built up decks, flat roofs are a very similar scenario, the space above the ceiling is a plenum but is really a sealed attic
    Last edited by Carnak; 07-17-2008 at 11:44 PM.

  2. #15
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    Jan 2004
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    Lancaster PA
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    Would having gable vents do the same?
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  3. #16
    Join Date
    Aug 2006
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    Dallas (Plano), TX
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    Quote Originally Posted by andserco View Post
    ... IMO, your attic is now a return plenum, so is it fire rated ? ...
    For my single-story 'ranch style' residence with a large attic / high roof:

    With my prior HVAC system, I had an Enerstat 3+2 (5 zone) controller, with its smoke terminals connected to my fire detection system. If a smoke/heat detector went into alarm, the fire alarm panel would close the circuit to the HVAC zoning panel's smoke terminals. This action would cause the zoning panel to shut down the HVAC equipment and close all damper motors.

    With my current HVAC system, the 'smoke terminals' are on the HVAC unit itself now, and are connected in the same manner to the fire alarm system. Same action if a smoke/heat detector goes into alarm - HVAC equipment shuts down, pumps stop, and damper motors close.

    For an attic that is conditioned air that is part of the system of breathing air for a home, I would take a careful look at strategies to ensure smoke doesn't enter into the living areas of the residence. A well thought out and aggressive smoke detection system in the attic, for an area that is conditioned air, I would think should be a must.

    Best regards,

    Bill

  4. #17
    Join Date
    Dec 2006
    Location
    Dallas
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    189
    Back to the original post:
    ...He, however, indicated that I wanted enclosed return; he would modify the system without charge...
    I think I would go with this option.

  5. #18
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    Apr 2002
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    Quote Originally Posted by beenthere View Post
    Would having gable vents do the same?
    not following gable vents, talking about a sealed attic

  6. #19
    Join Date
    Jul 2002
    Location
    Yuma, AZ
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    2,361
    The sealed attic is great with return and supply duct inside of the thermal envelope of the building.
    If there is no return duct, the concept is that the attic space becomes a return air plenum and this is not uncommon in commercial plenums.

    • I have a huge concern about fire safety. As a contractor I would not consider doing that unless the National Fire Protection Association (NFPA) approved the proceedure http://www.nfpa.org/index.asp Talk to your local fire departement (It is their job to know the local codes. I don't happen to know our local position but I will look into it.)

      I would not want the responsibility or liability for property loss or loss of life in case of a fire propagated or spread through an unducted attic return air system,

    • Smoke generated by attic contents could also be a major safety concern. The foam, plastics and other items may produce toxic fumes and spread them throuhout the house along with the fire.

    • Finally the attic is sealed the day it is built. Will it be sealed after the plumbers, cable guys, and other home modifiers work in the home over the next 30 years? If not what will that do to the power bill and the home itself if incoming air brings moisture and mold problems into your attic space? This concern alone would make me unwilling to inovate. Entire homes have been destroyed by runaway moisture and mold issues. Mold insurance is out of the question...unaffordable. That makes me think that you may also want to talk to your insurance company about this matter.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  7. #20
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    Quote Originally Posted by Carnak View Post
    not following gable vents, talking about a sealed attic
    Was just wondering if installing gable vents would be enough ventilation for the insulated attic.
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  8. #21
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    Quote Originally Posted by beenthere View Post
    Was just wondering if installing gable vents would be enough ventilation for the insulated attic.
    It is self defeating - the key is a sealed attic

    You vent attics to prevent icycles in Canada and Pennsylvania.You do not worry about icycles when you are running cooling.

    It would be like teddy's experiment where he leaves a window open.

  9. #22
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    Quote Originally Posted by Carnak View Post
    It would be like teddy's experiment where he leaves a window open.

    Ouch. LOL..


    I was thinking(bad thing sometimes) since its sealed and insulated. The vents would keep it at OD ambient. Instead of a regular attics 130.
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  10. #23
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    Quote Originally Posted by beenthere View Post
    Ouch. LOL..


    I was thinking(bad thing sometimes) since its sealed and insulated. The vents would keep it at OD ambient. Instead of a regular attics 130.
    I got a good graph here, was my attic during a time frame last August. I was not conditioning it then. We had Hurricane Dean go by in this time frame but we were only getting tropical storm force winds, steady 40 to 50 mph, some gusts into the 60s. Power was out for about 8 hours





    So over that time frame the attic averaged 80.5 F dry bulb, 46.7% RH, dewpoint of 58.3

    Attic air tmps floated from 78 to 84 F. I marked some of the temperature spikes as "1" and these all occur after the sun has set.

    I got a time frame marked as Number "2", and what I was doing here was running my AC 24/7 chilling the whole space below down to 70F. It is all concrete and I figured I would be stuck inside for a long time without power so I chilled my thermal mass down. It was fairly sunny while I was doing this, and the attic only got down around 78 degrees. The attic dewpoint was down around the minimum the stats show, and the RH plummetted up there.

    The time frame I marked as "3" was when they shut all the power down as the storm started to blow. It was obviously cloudy, the attic temperature climbed only a little bit, the RH sailed up to 50%, dewpoint rose. Pretty good considering it was like having a blower door test being run for 8 hours.

    Area marked as "4" is after the power came back on maybe 8 hours later. The humidity drops in the attic, moisture diffuses through the dry wall ceiling to the space below.

    I later added a small amount of supply air to the attic space, it averages 79 and RH in the 40s.

  11. #24
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    Aug 2006
    Location
    Dallas (Plano), TX
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    Quote Originally Posted by Carnak View Post
    ... dewpoint of 58.3 ... dewpoint rose ...
    Very interesting, and very helpful. Thanks!

    How did you arrive at dewpoint - measured it directly, or calculated it from DB Temp and RH?

    If you measured it directly, what did you use? Or if you calculated it, how did you do it?

    Many thanks.

    Best regards,

    Bill

  12. #25
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    Apr 2002
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    dewpoint is a calculated value, the software that you use with the data logger calculates it

  13. #26
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    Apr 2008
    Location
    Texas
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    As you can tell from the variety of responses you are going outside the intended function of your attic.

    First off, if you seal your attic with foam insulation then it becomes a sealed unconditioned portion of your home. ANY exterior penetrations to “Vent” the attic is pointless. ANY vent penetrations of the attic will defeat the entire purpose of sealing the attic with foam.

    What was the purpose of installing a return duct in the attic? Technically, there is no such thing in HVAC as a “fresh air return” anyway.
    Are you wanting an outside air duct to bring outside (fresh) air into the house? Why do you need a “fresh air duct”? How long have you lived in Texas?

    I think from the reaction of your inspector you can see his concern about a “fresh air return”. What are you trying to accomplish?

    You said, “My HVAC contractor provided me several names of other builders that use the open fresh air return in the attic with great success”
    Success doing what???????
    As far as a “Green tag” from the city, well…. Tape a dollar to it so it will be worth something.

    The real question I have is what do you want to accomplish with your HVAC system?

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