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  1. #1
    Join Date
    Aug 2012
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    3

    Default A/C duct sweating

    We have 2 a/c ceiling vents. One in each bathroom. Each is on an interior wall. There is an insulated un-heated attic above. Occasionally we find condensation on them. It doesn't matter if the bathroom is used or not.
    When the showers are used we always put on the exhaust fan.

    Any ideas how to stop the sweating?

    Thanks.

  2. #2
    Join Date
    Jul 2010
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    SoCal
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    5,084

    Default Re: A/C duct sweating

    Try insulating them or replace with flex ducts.

  3. #3
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: A/C duct sweating

    What exactly is sweating, the ducts, the vents or both? If its the ducts, are you seeing water on the inside of the duct or the outside? Is it only bathroom vents and ducts or are other rooms affected as well? Do you see sweating on the outside of the ducts in the attic?

    Your attic insulation, is it in the ceiling or have you insulated the rafters and the gable ends? Does the attic have adequate ventilation?

  4. #4
    Join Date
    Aug 2012
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    3

    Default Re: A/C duct sweating

    We have flex insulated ducts, metal boots. The attic floor is insulated. The condensation is on the vent grilles.
    Thanks

  5. #5
    Join Date
    Jul 2010
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    SoCal
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    Default Re: A/C duct sweating

    Quote Originally Posted by cashm View Post
    We have flex insulated ducts, metal boots. The attic floor is insulated. The condensation is on the vent grilles.
    Thanks
    3 things to check now: your return, the filter and room humidity.

    Question: what temp is your thermostat set on and what's the temp outside?

  6. #6
    Join Date
    Aug 2012
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    3

    Default Re: A/C duct sweating

    It's 90 outside & Stat is set on 79.
    We even have condensation in a guest bath that has not been used.

  7. #7
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: A/C duct sweating

    You may not be able to do much about it. Here is a link to an absolute humidity (AH) vs, relative humidity (RH) vs temperature (C). It comes from the German government so its in Celsius instead of Fahrenheit.

    http://www.tis-gdv.de/tis_e/misc/klima.htm

    The air coming out of the ducts to any room is somewhere in the 40F to 50F or about 5C to 10C. The 79F that your thermostat is set to is closest to the 25C (77F) on the chart.

    The air in the duct is going to be near 100% RH, any more moisture has condensed out at the cooling coils (evaporator coil). That air is it 10C (50F) it has an AH of 9.4 grams of water per cubic meter. Thats all it can hold. It is no doubt cooling the surface of the vent down to pretty close to that temperature.

    Now if the room air is at 77F or 25C and 50% RH, which is a pretty typical RH in most homes, that air has an AH of 11.5 grams per cubic meter (g/m3). That is more water that the air at the vent can hold, so the excess water vapor is going to condense on the grate of the vent.

    Typically, a bathroom, even unused will have a higher RH than the rest of the house because of the toilet bowl. Bathrooms tend to have a greater temperature swing than other rooms in the house because of their relative small size.

    A big factor in the RH inside your house is the duty cycle of the AC. Duty cycle is the %on/total time. The longer the duty cycle, the lower the RH as is is constantly dehumidifying the air. An AC that only runs for a few hours a day or a few minutes at a time with a long off time is oversized for the application, and because it is off more than on, the RH inside the house will be higher, especially when the outdoor RH is high.

  8. #8
    Join Date
    Jul 2010
    Location
    SoCal
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    5,084

    Default Re: A/C duct sweating

    Quote Originally Posted by cashm View Post
    It's 90 outside & Stat is set on 79.
    We even have condensation in a guest bath that has not been used.
    Measure your indoor humidity and if too high try a humidifier in the room with the most condensation and see if it helps.

  9. #9
    Join Date
    Jul 2012
    Location
    Manhattan
    Posts
    17

    Default Re: A/C duct sweating

    Ducts sweat because the outside surfaces of the ducts are below the dew point of the air in their immediate surroundings. The primary ways to stop ducts from sweating is to raise the surface temperature of the ducts or to lower the dew point of the air surrounding the duct by some form of dehumidification.

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