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  1. #92
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    hmmmm.

    Does an airplane fly because of the air pressure below the wings, or because of a suction above them.

    If I remove a sidewall supply, and put a piece of paper in it, and put the grille back on, will the paper blow against the grille because of pressure in the duct, or the return sucking air out of the room.

    If I win the powerball for 50 mil, will I give a 2 week notice, or just not show up.
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  2. #93
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    Try another Mr. Wizard experiment.

    Get a metal can with a small screw on top. Remove top. Heat it up. Screw on cap and remove from heat source.

    As can cools it starts to collapse. Why?

    You can pull on a rope, you can pull your wire put you cannot pull air, you have to push it.

    There is really no such thing is a negative pressure or a negative temperature, we just conveniently use the terms all the time.

    Either you have heat or no heat. Temperature starts at absolute zero degrees and goes up. You cannot transfer cold to something but heat can come and go.

    Either you have pressure or you do not, it starts at a perfect vacuum and goes up.

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  3. #94
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    Originally posted by dash
    Originally posted by RoBoTeq


    The return air portion of an HVAC ducting system does not "pull" or "suck" air...it allows the higher pressure of air introduced into an area to "return" to the units blower.


    "it allows the higher pressure of air introduced into an area "

    That sounds like what you have stated before ,that the supply air pushes the air to and/or thru the return.


    Question,if we have place paper over part of the return grille,it stays in place rather firmly.So is it being held their by the supply air pushing it,atmospheric pressure ,or suction from the fan????
    The room air pressure

    Hopefully in the type of climate you and I deal with it is slightly higher than atmospheric.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  4. #95
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    My, my... I wonder if the maiden wet her pants for that last tirade against my character

    Aside from the constant stupid interruptions by the maiden, this thread is bringing out a lot of answers to questions that seem to mostly be irrelevant until we get particular in the way we are viewing how physics works in our industry.

    To answer the question about the plane; the air below the wing pushes the plane up as the curved top of the wing creates a lower pressure area on the top.
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  5. #96
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    Originally posted by RoBoTeq


    To answer the question about the plane; the air below the wing pushes the plane up as the curved top of the wing creates a lower pressure area on the top.

    thanks for the answer, but I did know that.

    discoverly channel, one of the 4 channels i watch.

    Plus my dad took flying lessons.
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  6. #97
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    Originally posted by Carnak
    Originally posted by dash
    Originally posted by RoBoTeq


    The return air portion of an HVAC ducting system does not "pull" or "suck" air...it allows the higher pressure of air introduced into an area to "return" to the units blower.


    "it allows the higher pressure of air introduced into an area "

    That sounds like what you have stated before ,that the supply air pushes the air to and/or thru the return.


    Question,if we have place paper over part of the return grille,it stays in place rather firmly.So is it being held their by the supply air pushing it,atmospheric pressure ,or suction from the fan????
    The room air pressure

    Hopefully in the type of climate you and I deal with it is slightly higher than atmospheric.
    So at what point does this air that is pushing in the return grill change to negative pressure and what causes it to do so? If what you say is true there would be no negative pressure in return ducts because the air is being pushed to the blower.

  7. #98
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    Originally posted by beenthere
    Originally posted by RoBoTeq


    To answer the question about the plane; the air below the wing pushes the plane up as the curved top of the wing creates a lower pressure area on the top.

    thanks for the answer, but I did know that.

    discoverly channel, one of the 4 channels i watch.

    Plus my dad took flying lessons.
    I never flew a plane, but I have jumped out of nearly 300 of them. The parachute I use is actually a square, semi-rigid wing that works on the same aerodynamic principles that all wings work from.

    As carnak stated; there is no "negative" pressure associated with blowers in HVAC. There is high pressure areas and low pressure areas. Air always moves to a lower pressure area by the air pressure from the higher pressure area pushing its way into the lower pressure area. A blower is designed to mechanically move amounts of air from one side of the blower to the other. This pushes the air on the side that the air has been moved to and the air is replaced on the other side by higher pressure air pushing into the place where the blower took the air from to push.
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  8. #99
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    When you take an esp reading before the blower with an incline manometer, isn't it referencing to the pressure out side the duct,(atmospheric pressure) trying to push through the manometer.

    Its a pressure differencial.

    Some one will correct me, but atmospheric pressure is about 411.6 IW, So a -.4IW returm would be 411.2 IW, its still pressure, just less.

    Your measuring the loss of pressure, not a vacuum or suction.



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  9. #100
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    you have two static presures. supply and return
    return is not a negative presure it is a positive presure to the blower. the supply is a positive pressure leaving the blower. if you increase the static pressure to the blower you will get less presure on the supply. if you decrease the static on the return and alow more air to enter the blower you can increse the static in the supply
    in either case you are useing the same air from the eara in witch the air is being used.

    to have a negative presure you need to take the air from one place and exhaust it to another with out that same air going back to the same eara it is being taken from


    we are useing the same air in the same total eara and the
    return takes it in and the supply puts it back at differant volumes and cfm

    you can have more return grilles then needed as long as you maintain the proper static presure in the return trunk line. the more grilles the easer it is to have the proper return.
    imo only

  10. #101
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    Originally posted by beenthere


    Your measuring the loss of pressure, not a vacuum or suction.

    This is exactly why you add the negative return reading & positive supply reading when performing a TESP test & disregard the symbols.
    These are still absolute pressures the fan is seeing.
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  11. #102
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    So we would assume that a home with all sealed air ducts with no leaks would be at atmospheric pressure also? What allows us to pressurize a building with outside air that should be at the same atmospheric pressure as inside the building?

  12. #103
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    Originally posted by trane
    So we would assume that a home with all sealed air ducts with no leaks would be at atmospheric pressure also?


    Your return will have a reduced pressure, and the supply will have an increased.


    What allows us to pressurize a building with outside air that should be at the same atmospheric pressure as inside the building?
    Vapor compresses.
    T he return grilles are restrictions, the return duct has a reduced pressure so the outside air pushes into the return duct, giving the blower more airthen the house can contain with out having a small amount of compression(positve pressure).
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  13. #104
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    Also, this is being done with a structure that is not air tight. If a home were completely air tight a blower would not be able to bring in more air then the supply side is producing. It must equal out.
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