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  1. #1
    Join Date
    Nov 2007
    Location
    Gulf coast of Florida
    Posts
    5

    Did I do this to myself ? Duct sweating...

    Scene of the crime, 1981 block home, 1500 sq ft under air, Air con system was probably as old as the house, circa 1981.

    Summer of "05 one main duct started sweating and stained the ceiling. The other main duct going to the other side of the house stayed dry.

    This was the first time in our four years in the house this had happened. I at first thought it was because we had a large get together the week before and the system ran for several hours straight.

    So last winter I patched the ceiling (btw I hate popcorn ceilings), and replaced the insulation in that part of the attic (I'm not so fond of fiberglass batts either)

    This summer it happened again so I'm trying to figure out why after 24 years the ducts started sweating ? Then the light bulb went on. I had closed three out of six ducts on that side of the house to force more cold air into the master bedroom (The Mrs likes it REALLY cold when she's sleeping).

    I suspect that closing the vents caused that duct to chill down so much it started sweating. Is this a possible scenario or do I need to look into other causes ?


    Thanks for your time,

  2. #2
    Join Date
    Sep 2003
    Location
    McKinney, TX
    Posts
    470
    Quote Originally Posted by Agu View Post
    Scene of the crime, 1981 block home, 1500 sq ft under air, Air con system was probably as old as the house, circa 1981.

    Summer of "05 one main duct started sweating and stained the ceiling. The other main duct going to the other side of the house stayed dry.

    This was the first time in our four years in the house this had happened. I at first thought it was because we had a large get together the week before and the system ran for several hours straight.

    So last winter I patched the ceiling (btw I hate popcorn ceilings), and replaced the insulation in that part of the attic (I'm not so fond of fiberglass batts either)

    This summer it happened again so I'm trying to figure out why after 24 years the ducts started sweating ? Then the light bulb went on. I had closed three out of six ducts on that side of the house to force more cold air into the master bedroom (The Mrs likes it REALLY cold when she's sleeping).

    I suspect that closing the vents caused that duct to chill down so much it started sweating. Is this a possible scenario or do I need to look into other causes ?


    Thanks for your time,
    Yes this is a possible cause. Systems are designed to pass a certain amount of air, so when you damper down ducts or close grilles it could create this problem. You have possibly raised to air pressure (static) in your system causing a problem. I would also look to make sure that the insulation has not fallen off of the duct that is leaking and that it is still in tact. BUT the best solution would be to call a professional and get it properly fixed, that way you wont keep wasting time repairing your ceiling and damaging your unit. If the Mrs wants more air into her room, maybe the ductwork should be modified to make that happen. shutting off ducts is not the answer.
    Just call a contractor, its not that expensive for a service call to have someone check it out.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,710
    What harris said.

    Also check/ er have checked, the insulation around the duct, it might have mold from getting soaked.
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  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Could be a couple of different things;

    The outside of the duct insulation needs to be "vapor sealed" or moisture will condense in the insulation ,rendering the insulation useless and cause sweating.

    Low air flow due to dirty coil,blower,filter,etc., will cause air to be colder then designed for,and ducts will sweat.

  5. #5
    Join Date
    Nov 2007
    Location
    Gulf coast of Florida
    Posts
    5
    I've been doing a ton of reading here trying to figure out my problem. I've already decided the insulation needs to be replaced, again.

    Since the original sweating incident I've made two significant changes. All windows and sliding glass doors (single pane and leaked like a sieve) were replaced with new insulated glass units. In addition the antique compressor and air exchange units were replaced by a professional. I now have a Trane 3 ton XL16i compressor and matching air handler. My electricity usage dropped by 40% and the air con barely runs compared to the old system. However the vents were still closed off, and the ductwork still sweated.

    Will a static pressure test determine if my ductwork is adequate now that the vents are open?

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,710
    Yes, it can tell you if your duct work is sized ok or not.

    Also, ask your contractor, what cfm per ton he has it set for. Too low can also cause this even if the duct is sized right.
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  7. #7
    Join Date
    Sep 2003
    Location
    McKinney, TX
    Posts
    470
    you really need to stop trying to figure this out yourself and get your contractor involved.

  8. #8
    Join Date
    Nov 2007
    Location
    Gulf coast of Florida
    Posts
    5
    Quote Originally Posted by a/c-harris View Post
    you really need to stop trying to figure this out yourself and get your contractor involved.
    Tried that already and called the company that installed the system. He came in, looked at the spot in the ceiling, and declared I needed all new ductwork and returns. He said closing off three vents wasn't a problem. He also said running an additional vent/return wasn't an option because my ducts were "leaking".

    Did I mention he never made it into the attic, and that he never actually inspected or tested anything. He doesn't do installs so he never was in the attic for the original install.


    BTW, they weren't low bidder on the original job either.

  9. #9
    Join Date
    Nov 2007
    Location
    Washington State (on the Peninsula)
    Posts
    80
    open the vents

  10. #10
    Join Date
    Nov 2007
    Location
    N.C.
    Posts
    170
    Quote Originally Posted by Agu View Post
    Tried that already and called the company that installed the system. He came in, looked at the spot in the ceiling, and declared I needed all new ductwork and returns. He said closing off three vents wasn't a problem. He also said running an additional vent/return wasn't an option because my ducts were "leaking".

    Did I mention he never made it into the attic, and that he never actually inspected or tested anything. He doesn't do installs so he never was in the attic for the original install.


    BTW, they weren't low bidder on the original job either.
    That's crazy. They should have check the ductwork in the beginning of the install. He should've mention all of the things he is saying now before you had to do some work to your ceiling. Especially for a 20-plus year system.

  11. #11
    Join Date
    Nov 2007
    Location
    panama city beach (aka lower alabama or LA) FL
    Posts
    93

    ?

    is this the same guy who replaced your old equipment? if he is then whydidnt he adress this at the time of the install, if hes not then why didnt the contractor who installed the new system adress the leaking ducts ? and you should ask which comic book back he got his x-ray glasses from so i can get me some i been looking for a way to look at the ducts in the attic without actually getting in the attic would save me lots of time..

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,258
    [QUOTE=dash;1684108]Could be a couple of different things;
    The outside of the duct insulation needs to be "vapor sealed" or moisture will condense in the insulation ,rendering the insulation useless and cause sweating.
    QUOTE]
    This is excellent advice! A/c ducts are normally below the dew points of the air in unconditioned space. Without the vapor barrier on the outside of the insulation, the vapor from the high dew point air migrates to the cold duct surface. Colder ducts and higher dew points require more insulation with a perfect vapor barrier.

    Quote Originally Posted by dash View Post
    Could be a couple of different things;
    Low air flow due to dirty coil,blower,filter,etc., will cause air to be colder then designed for,and ducts will sweat.
    Yes, raising the air flow raises the duct temperature but decreases the moisture removal of the a/c. Regardless what method of raising air flow, expect less dehumidification from the a/c. Many a/c operate succesfully because of restricted ducts that lower the a/c coil temperature. The a/c removes adequate moisture for the wrong reasons, but works. Better to have large ducts, low pressure drop filters and a lower fan speed. Search for "Kevin O'Neil" for a through discusion on this problem. Insulated ducts close to other cool surfaces like other cold ducts or cool drywall tend to sweat on the outside of vapor barrier surface. Season greetings, TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
    Join Date
    Nov 2007
    Location
    St. Louis, Mo
    Posts
    28

    Red face

    dido open the vents

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