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  1. #1
    Join Date
    Jun 2004
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    Let me ask a question which probably is way out of the norm for professionals. Let's say there is a 2-story house which has an uncomfortably big temperature difference from top to bottom. If one made a duct from top to bottom and powered air thru it with a fan, could that possibly work?

    I understand a superior way would be to apply Manual D and do ductwork the *right* way. But for this question I would like to assume that will not be done. If possible I would like some help in understanding the pros and cons of doing a "hillbilly engineering" job like this.

    While my question is directly about temperature, in my mind I am also wondering whether this might apply to humidity issues. With that direction it is far less clear that Manual D holds the answers.

    Any help you can offer with this incredibly naive question, would be much appreciated!

    Thank you -- P.Student

  2. #2
    Join Date
    May 2004
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    11,967
    PS

    What ya do'n now man?

    Is this really theory or are you going to live above the garage?

    No, I am not going to start in on ya. Just was courious if you were going to apply this to a situation that you have.


  3. #3
    Join Date
    Nov 2000
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    Eastern PA
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    68,923
    I think this is a very good question for specific problem applications.

    I have done remodeling of many large three story row homes that were originally built before mechanical cooling was in use. These homes have larger central staircases with operable skylights at the top of them to draw air through open windows up the staircase and out the skylight openings.

    With the age of mechanical cooling and the fact that these homes are in the middle of areas that you don't want to be leaving windows open in, the design creates a thermal problem.

    The top of these staircases get extremely hot due to the chimney effect of this design. Even with seperately zoned cooling the top floors of these homes suffer from the heat rising in these staircases. I had installed a central return shaft to the top of these staircases with the central return grill at the highest point of the home. During any cooling or heating cycle of the lower floor systems the hot air from the top of the shaft would be returned to the central system to be conditioned. This mechanically recreates the effect that the skylight was intended for.

    On many of these systems I would install a simple temperature sensor set at 90ºF at the return grill that would close the fan circuit to the furnace. This way the blower greatly reduced thermal stratification even when the system was not calling for cooling. It also helped greatly with the heating.
    Government is a disease...
    ...masquerading as its own cure…
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  4. #4
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    3,304

    Imaginary only at this time

    I do all kinds of things in my head that I would be afraid to do in real life. This is one of those ideas that I just cannot stop thinking about. If I really understood HVAC as well as I wished, I would be able to discuss what would happen with such a transfer setup -- but I don't know that much.

    Does the example make sense to you as a question?

    Best wishes -- P.Student

  5. #5

    dumbest question ever?

    Pretty commnonly done, but not worded like you did. CNN Center in downtown Atlanta uses a large shaft to return air from the top of the atrium. Your ceiling fan in a tall room or split-level home does the same thing.

  6. #6
    Senior Tech Guest
    I used to say the only dumb question was the one you didn't ask...and then I registered here...
    But yours isn't a dumb one...from fresh thoughts and ideas patents and wealthy men are born.

  7. #7
    Join Date
    Jul 2004
    Location
    West TN
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    983
    Your question made me think of something I used to wonder about.

    This is theoretical of course.

    Summer time
    Moisture vents to crawl space serve as fresh air in.
    Brick homes have a gap between the brick and the stud wall.
    Use that space as a 'ducting' to the attic.
    Attic does NOT have vents in overhangs.
    Powered exaust fan in attic.


    So fan from attic pulls cool air from under house,
    up along inside cavity of outside walls, then into the
    attic to be vented out.

    This would provide cooler air to attic for ventilation.

    Used to try to figure up all the downfalls to the application.
    Such as Fire Hazard. Really old houses that had the studs
    without a top plate give flames a way to spread rapidly vrs
    todays houses with the top plate design.
    i.e. Studs 10ft tall but ceiling is 9ft tall.
    There's 1ft of stud sticking up in the attic.
    With no insulation in outside wall, you could get up in attic and look down in the wall.
    Like I said, this design helped flame spread rapidly from in the wall to the attic or upper floor levels.


    Another drawback is moisture.
    But this is one that is pretty tricky.
    Brick walls serve as moisture capacitors.
    Bricks are sorta like sponges... they take in the moisture from outside then transfer it to the cooler side.
    This is why its so important to have a moisture barrier
    on the outside walls. i.e. Tyvex

    But where does this moisture go without ventilation?
    Of course the answer there depends on all sorts of things,
    such as temp of the walls with the Tyvex on them and temps
    under house. I imagine alot of that moisture remains trapped in that space since there is no air movement to
    transfer it anywere. The only airmovement would be air currents from under house when the moisture vents are open.




  8. #8
    Join Date
    Mar 2005
    Location
    Suppy NC
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    4,513
    take the total size on the system. see what you need for the first floor and the second floor. add a little more for the second floor and size the duct to the load for each floor. run two trunk lines and if possible run second floor to the attic then the take offs from there.
    put more return on second floor and balance out the system
    you idea in theory should work to some extent but sizing is what is most important. when a single trunk line is ran in a lot of cases the first floor steals the air need the feed the second floor. two trunks seem to work better and if the first floor is a little under the second will get it
    this will alow the system to run longer pulling out more humidi and also help in cooling the second floor because you are giving it the air it needs at the same time pulling out the hot air.
    to just run a duct with a fan in it will be the same as running the blower all the time and will raise the humidity and raise the electric bill to
    man-j and d are needed and duct sizing is very important
    this can also be done with two trunk line in a basemant to just a little harder and more thought involved takeing in acount the lenth of the take offs to the second floor
    static presure and volume are very important factors here

  9. #9
    Wormy, when I hit the field, I had simular idea.
    I spoke with an ol fart and he set me straight.

    That cooler area underneath the home would soon become hot due to the drawing in of warmer air from the outside.
    And this ouwl occur due to the drawing of the cooler air up into the attic space.


    You would need a cave the size of a foot ball field in order to draw cool air from in order to satisfy the demand from the attic.
    Plus, without a radient barrier upstairs, this cool air would shortly become hot itself.


    Do some research on Frank Loyd Wright. He built a home on the Arizona desert which was half buried into the hillside.

    Lots of windows. No mechanical A/C though and the placee stayed cool year round.

    There are few today who are smart enough to live in a house designed like some of his were.

  10. #10
    Join Date
    Jan 2004
    Posts
    108
    Drag out the calculator. Assume a duct and blower sized for, say, 1000 CFM. If the temperature difference from top to bottom is ten degrees F, then that duct will move 10,800 BTUH from the hot floor to the cooler floor. Whether this is enough to make a significant difference depends on the difference in net heat gain or loss for the two floors.

    Humidity seems to be easier. My limited experience is that the absolute humidity will be nearly constant through the entire house, especially if the house is fairly airtight.

  11. #11
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    Do some research on Frank Loyd Wright. He built a home on the Arizona desert which was half buried into the hillside.
    Frank Lloyd Wright architecture is one of my hobby/interests. Not only did he design a home in AZ as you say, but another one near Madison, Wisconsin for the Jacobs family, partially buried in a berm to the north yet open to the sun on the southern facade. Although the emphasis for the Madison design was for solar heat, I'm sure it has a lower cooling load in summer due to much of the home's walls being below grade.

    In my younger days I was hell bent to build an earth sheltered house. Had the land and plans and everything. I was in a community where everyone else was vowing to do the same thing ( a bunch of held-over hippies, as it turned out), using a construction material known as ferro-cement. Of the few houses that actually did get built (mine was not among them), the biggest lessons learned was that any section of the structural envelope of the house that was below grade had to be absolutely water tight, and insulated. Turns out mother earth is actually a lousy insulator, robbing heat from the interior during winter. Summer time is a different story...low solar gain and insulation slows even further any thermal lag. Only problem is dehumidification...a dehumidifier would work well in this kind of house.

    As for P. Student's question, it is not dumb at all. For me it touches on how I think regarding residential construction in Texas (since both P. Student and I live in the lone star state). For starters, two stories is out unless the lot size dictates otherwise. Additionally, standing seam roof (insane to have asphalt shingles where it hails), no attic at all (highly insulated rafter bays with ventilation from soffit to ridge vent instead), all ductwork for HVAC in conditioned space, broad overhangs on roof to carry rainwater away from foundation and provide shading to the walls exposed to solar heat gain in summer, reasonable interior ceiling heights, all exterior siding able to withstand years of exposure without significant wear (brick, stone, stucco, etc), and so on.

    Frank Lloyd Wright was ahead of his time in that many of his "Usonian" designs incorporated several of the aspects I listed above. Too bad it's not vogue among home builders (or buyers) to incorporate some of these elements into modern construction.
    • 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.

  12. #12
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    3,304

    Couple issues

    Wormy said:
    >>Summer time
    >>Moisture vents to crawl space serve as fresh air in.
    >>Brick homes have a gap between the brick and the stud wall.
    >>Use that space as a 'ducting' to the attic.
    >>Attic does NOT have vents in overhangs.
    >>Powered exaust fan in attic.

    Are you familiar with the type of wall construction known as "skin venting" or "vent skinning"? This has some similarities with what you describe, however the air intake is on the outside near the ground, not the crawl space. And nowadays used with a ridge vent so the stack effect powers the ventilation, not a power fan. Additonally, they usually work with vents in the overhangs just like most houses.

    So, does this resemble your idea or not so much?


    Engineerguy said:
    >>Humidity seems to be easier. My limited experience is that the
    >>absolute humidity will be nearly constant through the entire house,
    >>especially if the house is fairly airtight.

    Hmmm. How strongly do you expect that humidity will be nearly constant? In my own house I keep a little meter in various places and see about 10 points higher RH in each bathroom. As far as I know there are not any undiscovered water leaks or other abnormalities to account for this.

    At this point I must admit that reducing bathroom RH is one hypothetical application that I am thinking about (am fully aware that beforehand must have good vent fans, etc.).
    Wondering whether maybe 75 CFM continuous feed into the bathroom from a drier area maybe 10 feet away, would be effective in achieving that goal. And what would be necessary to avoid any bad effects, most especially any that I have not though of yet.

    Best wishes -- P.Student

  13. #13
    Originally posted by perpetual_student
    Let me ask a question which probably is way out of the norm for professionals. Let's say there is a 2-story house which has an uncomfortably big temperature difference from top to bottom. If one made a duct from top to bottom and powered air thru it with a fan, could that possibly work?

    Thank you -- P.Student




    Would it work? Yes, and a reversible fan motor to switch for each season would be effective but duct sizing would still be a factor. If the trunk was sized improperly, the velocity of the air moving to and from is a huge consideration to overlook.
    You would be removing air from a conditioned space and replacing air into another conditioned space with a control serving only one level for starting and stopping.

    Would it work effectively? No, and a dual zoned system with seperate ducts and controls would be more efficient.
    The heat gain/loss considerations remain the same.

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