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  1. #1
    Join Date
    Aug 2008
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    107

    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
    Location
    North Carolina
    Posts
    428
    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
    Location
    Poestenkill, NY
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    769
    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
    Join Date
    Aug 2008
    Posts
    107
    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
    Join Date
    Jul 2008
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    Poestenkill, NY
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    769
    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
    Posts
    11,086
    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
    Lancaster PA
    Posts
    66,808
    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
    Mar 2001
    Location
    North Carolina
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    428
    Quote Originally Posted by beenthere View Post
    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.
    I read it as air being sucked out of the attic access door
    Which i assume attic air was being sucked into the living space.

    And i don't think we should be telling him to dismantle any part of the system as he should call out a pro to evaluate the problem.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    So you think opening the return air filter grille, and looking to see if it, and the return box are sealed, is too hard to do?
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  10. #10
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    769
    Quote Originally Posted by beenthere View Post
    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.
    Yeah, I read it as sucking the other way, since he also mentioned "sucked under the front door".

  11. #11
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    Quote Originally Posted by beenthere View Post
    So you think opening the return air filter grille, and looking to see if it, and the return box are sealed, is too hard to do?
    Right - please tell me this site isn't that petty....

  12. #12
    Join Date
    Jan 2004
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    Lancaster PA
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    Yes we are. LOL
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  13. #13
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    Quote Originally Posted by beenthere View Post
    Yes we are. LOL
    Sometimes a smiley goes a long way...

    As long as i'm totally high-jacking this thread - how do I go about becoming a professional member? Should i scan in my dimploma?

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