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  1. #1
    Join Date
    Aug 2008
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    Negative pressure

    My house is under negative pressure when the a/c is on. I can feel air being sucked out of the attic access door. I'm assuming it is being sucked under the front door as well. I checked the duct work in the attic and didn't see any noticeable air leaks. What else would cause negative pressure?

  2. #2
    Join Date
    Mar 2001
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    North Carolina
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    low supply air, bath fans running, drier running anything that exhausts air with out providing the same amount of air it exhausts. in your case most likely not enough supply air vs return air. if your return pulls 1000 cfm and you only supply 800 cfm your under negative pressure. this can be due to supply air duct leakage which would be the most common reason found.

  3. #3
    Join Date
    Jul 2008
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    Poestenkill, NY
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    Re-check the supplies in the attic. Confusios say, "If the problem is only when the AC is on, it's from the AC system."

    The leaking supplies are causing an imbalance - the AC return (I assume) is directly connected to the living space while the supplies are ducted and leaking in the attic - wa-la - negative pressure in the living space.

  4. #4
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    Quote Originally Posted by larobj63 View Post
    Re-check the supplies in the attic. Confusios say, "If the problem is only when the AC is on, it's from the AC system."

    The leaking supplies are causing an imbalance - the AC return (I assume) is directly connected to the living space while the supplies are ducted and leaking in the attic - wa-la - negative pressure in the living space.
    Yeah, it only happens when the air handler is on. I checked all the joints and seams and did not find any air leaks. I'll check again.

  5. #5
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    Quote Originally Posted by bluefitness View Post
    Yeah, it only happens when the air handler is on. I checked all the joints and seams and did not find any air leaks. I'll check again.
    Do it. You're wasting energy by cooling the attic.

    Does the attic stay pretty cool? That's a sure sign you're leaking up there.

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    Find a supply vent grill in the ceiling and pull it off. If there's a gap between the supply vent sheetmetal (called a "boot") and the drywall, that is part of your problem. This gap can be sealed with caulk or duct mastic (latter is preferable).

    When the supply grill is on, and this gap between ceiling drywall and boot remains, back pressure off the grill will force cold air into the attic, which equates to air lost from the home. If it escapes the conditioned space, it is a loss of air volume, which has to be made up from somewhere.

    You should also consider ways to seal your attic access, and all other penetrations through the ceiling into the attic.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
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    If the supplies are leaking into the attic. He won't feel air being drawn into the attic through the skuttle. Might feel warm air being forced into the house.

    Also check seal at return where it penetrates the ceiling.
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  8. #8
    Join Date
    Apr 2002
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    Quote Originally Posted by bluefitness View Post
    My house is under negative pressure when the a/c is on. I can feel air being sucked out of the attic access door. I'm assuming it is being sucked under the front door as well. I checked the duct work in the attic and didn't see any noticeable air leaks. What else would cause negative pressure?
    attic air enters the space below or cold air goes up into the attic?

    cold air leaks out your front door or humid air is sucked in through the door frame?

    Is this a single story or two story house?

    Are the supply and return grilles in the ceiling?

    Is there a central return grille off of a hallway near the attic access?
    Last edited by Carnak; 08-13-2008 at 10:47 PM.

  9. #9
    Join Date
    Aug 2008
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    Quote Originally Posted by Carnak View Post
    attic air enters the space below or cold air goes up into the attic?

    cold air leaks out your front door or humid air is sucked in through the door frame?

    Is this a single story or two story house?

    Are the supply and return grilles in the ceiling?

    Is there a central return grille off of a hallway near the attic access?
    The house is under negative pressure, so hot attic air and outside air are being sucked into the house from the attic access and underneath the door. It is a single story house. The supplies are in the ceiling. It has a free return with a return transfer system (flex run from one room to the next). The attic access is in a far corner of the house, and the air handler is in the middle of the house. The are probably 30 feet apart.

  10. #10
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    Jul 2008
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    Poestenkill, NY
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    Quote Originally Posted by bluefitness View Post
    It has a free return with a return transfer system (flex run from one room to the next).
    Clarify that, please. The return is connected to the return grill with flex, right? What do you mean by "free return"?

    I still think the supply system is leaking somewhere regardless...

  11. #11
    Join Date
    Apr 2002
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    Quote Originally Posted by larobj63 View Post
    Clarify that, please. The return is connected to the return grill with flex, right? What do you mean by "free return"?

    I still think the supply system is leaking somewhere regardless...
    It sounds like a central return, then a transfer system.A return transfer grille in a room, and then a similar grille in the hall connected with flex, it is better than undercutting a door

    I would suspect a supply duct leak in the attic compounded by the central return scheme

  12. #12
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    Jul 2008
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    Poestenkill, NY
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    Quote Originally Posted by Carnak View Post
    It sounds like a central return, then a transfer system.A return transfer grille in a room, and then a similar grille in the hall connected with flex, it is better than undercutting a door

    I would suspect a supply duct leak in the attic compounded by the central return scheme
    Rigghhht - that makes sense.

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