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  1. #1

    Condensation on and in ducts

    Last summer I had a new AC installed and I have noticed recently that I had some water dripping from my ceiling. I thought it was from the AC drain or something but what I discovered is that a couple of my AC Ducts close to the AC Blower are either sweating alot or there is also some water in them.

    Can someone tell me if this is because I need new ducts that can handle the new AC or could it be a another reason?

    I was also running my fan on CIRC, does that increase humidity and condensation?

    thanks

    Mark

  2. #2
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  3. #3
    So I read through that thread but it did not help me much. I plan to call the HVAC company that installed the new system but the other thread did not say much about the solution. Does this mean that new ducts should be installed to increase air flow? Does this mean the existing ducts do not have enough insulation around them? Could this also mean I need another attic vent to bring down the temperature in the attic?

    I would like to get a little more information before I talk to the HVAC company because I expect they will just come back and tell me the AC unit is the right size and the problem is that I need new ducts.

    I would like to be prepared a little.


    thanks

  4. #4
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    Thumbs up

    Quote Originally Posted by RoadRashTX View Post
    So I read through that thread but it did not help me much. I plan to call the HVAC company that installed the new system but the other thread did not say much about the solution.

    Does this mean that new ducts should be installed to increase air flow? Does this mean the existing ducts do not have enough insulation around them? Could this also mean I need another attic vent to bring down the temperature in the attic?

    I would like to get a little more information before I talk to the HVAC company because I expect they will just come back and tell me the AC unit is the right size and the problem is that I need new ducts.

    I would like to be prepared a little. thanks
    I was trying to keep from typing as I have carpal tunnel in both hands & use eraser ends of pencils to type.

    It is strange that no one else responded to your thread.

    The temperature of the ducts is below the dew point of the surrounding air.
    Ducts should be insulated to R-6 with a vapor barrier on the outside next to the warm humid air.
    EDIT: Increasing Airflow through cooling coil - will help raise its temperature - toward lessening hitting the dew point level in the DUCTS.

    ...I discovered is that a couple of my AC Ducts close to the AC Blower are either sweating alot or there is also some water in them.
    Perhaps the cooling coil was iced over & the blower blew water into the ducts or, the blower is blowing condensate off the coil i to the ducts.

    We don't have enough info data on your equipment & other conditions too generate reasonably accurate - solution oriented speculations.

    Hopefully, when you provide a lot more information of the CONDITIONS involved, - shophound & others will do some typing work for you.

    Come on guys, anyone...
    Last edited by udarrell; 06-20-2010 at 11:54 AM. Reason: Airflow CFM through cooling coil & duct temps...

  5. #5
    Join Date
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    south louisiana
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    could be several things.
    first I would check the plenum at the duct collars to make sure that they
    are sealed. this is the first area with flex duct that requires a mastic seal.
    (not foil tape or duct tape..but mastic) when this area is not sealed
    or metal duct collar is exposed it will condensate. if the ducts are buried or
    lying on top of insulation they will condensate..where the ducts touch each other
    they will condensate. ducts should be sealed and strapped off the insulation
    with no sags or dips.
    the other leakage area is at the supply box. often these boxes are not flush
    with the attic flooring and hot air on the metal box and supply grill will condensate.
    while in the attic look where the supply box meets the attic floor. if you can
    feel a gap between the box and the sheetrock this is the cause of condensation
    on the grills. These gaps should also be mastic sealed. (not great stuff but masitc)
    any sizing of the ducts or problems with duct design or operation of the system
    will require more info.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  6. #6
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    Dallas & Longview, TX
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    Udarrell knows his stuff but I would like to clearify what I think he was saying regarding the duct temp. is that the outside of the vapor barrier that covers the insulation that covers the duct- needs to be above the dew point of the attic. This can be accomplished by increasing blower speed if it can be/needs to be done (this will decrease the ability of your house to be dehumidified by the Ac unit though). Checking on the ducting attachment to the plenum to eliminate the possibility of a stagnate takeoff that has too little airflow. Increase the amount of insulation to the existing ductwork. Checking refreg. charge to see if to low. Evap. coil could be clogged causing decreased flow thus decr. temp to duct..............

    You get my point- It can be so many things with each a set of possible solutions so we need more info or at least get a knowlegeable tech that can help you.

  7. #7
    Join Date
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    Answer These if you would.

    Is the air handler in an unconditioned space? (attic)
    Are the ducts only sweating by the air handler?
    Can you watch water drip from the duct or by where it connects to the air handler?
    Do you have the installation instructions?

    1. yes
    2. yes
    3. yes
    4. no

    If you answered as above, your problem is:

    The air handler is not installed at the correct pitch allowing the water to drain.
    This is one of those instances why everyone wants to know why a trap should be installed!!!You have to much water in the drain pan and the fan is tossing water droplets down the duct work.

    The fix:
    Call installing contractor,

    Have them check unit pitch
    Install trap
    Check to see if the unit should have a water shield installed.(this comes as an add on fix from the manufacturer)
    Always here

  8. #8
    Hey all, thanks for all the help. I dont know much about HVAC but when I was searching the web looking for answers I noticed that most flexible ducting I found on the web has a very thick amount of insulation around the duct. If I compare this to most of the ducting in my attic it seems to me that I really need to get the ducts replaced with something more efficient. The ducts in my attic were used on the old system and on most of them the thin plastic on the outside has totally disintegrated so that you can see the insulation. Plus I noticed that the insulation on the ducts dont seem to be more than 1/2 to 1" thick. These ducts are at least 9 or more years old.

    Any thoughts?

    I think if I call the HVAC company that installed the system to come out, the first thing they will say is that their system is installed correctly but that my ducts need to be replaced.

    Also, my answers to energy star's questions are exactly as mentioned.

  9. #9
    Join Date
    Oct 2008
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    483
    OP, I'm confused. You describe your ductwork in pretty bad shape, yet then you say it is around 9 years old. The way you describe those ducts, I'd say they are more like 35 years old.

    The outer layer is usually metallic but you describe it as "plastic"?
    You say its disentigrated. Metal doesn't usually rot.
    You say its a half to an inch thick, it should be at least an inch, but most liekly 2". 2" is ALL we allow in any design here, but thats commercial, so you res guys clear me up if I'm wrong.

    I simply don't think you can get an R-6 with under 2" of material. Maybe I'm wrong.

    My dads house was built in 1984, so his ductwork is 26 years old, my home is 19 years old, and none of the barrier is rotting like you describe.

    Whats happening is your vapor barrier seems to be gone, so moisture in the hot attic air is moving in close to your ductwork and touching it. That cold duct (with 60 degree air or less inside) is causing that moist air to drop quickly in temp to below the dew point and so water drops form. Since its already in your insulation, then its soaking the insulation and sounds like its sopping wet so now its dripping off.

    With a good insulated duct, for one, it keeps the hot away from the cold, and with a proper vapor barrier it keeps the moisture outside of that thick warm insulation. See if the insulation was too thin, but it had a good vapor barrier, then the actual surface of the insulation could fall below the dew point and still cause water drops to form. Thats why both parts are important.

    I'm surprised your installed put a new unit on ductwork like you described, because even though its not his fault, he had to know it was gonna result in getting callbacks on an issue outside his contract.

    You MAY not need new ducts, but it sounds like they need to be stripped to metal, and rewrapped with insulation and vapor barrier all the way down.

  10. #10
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    south louisiana
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    flex or hard ducting?
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  11. #11
    Energy and Rhizzle,

    The ducts are flex and not hard. I also looked in the attic again today and it looks like a couple of the ducts are more like Rhizzle described where the outer layer is metallic. So since our house was built in the 80's I will assume that the ones where the vapor barrier are gone are the originals and the ones that are metallic are ones replaced by the previous owners about 9 yrs ago.

    The ones where the vapor barrier gone are basically covered in a thin gray colored plastic and the insulation is thin as I described before. Rhizzle is right on that the insulation is wet on some of them close to the air box.

    I guess I am also surprised that the contractor did not even mention replacing them.

    So would it make more sense to get them re-wrapped? or to have them replaced completely?

    Thanks

  12. #12
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    Quote Originally Posted by RoadRashTX View Post
    Energy and Rhizzle,

    The ducts are flex and not hard. I also looked in the attic again today and it looks like a couple of the ducts are more like Rhizzle described where the outer layer is metallic. So since our house was built in the 80's I will assume that the ones where the vapor barrier are gone are the originals and the ones that are metallic are ones replaced by the previous owners about 9 yrs ago.

    The ones where the vapor barrier gone are basically covered in a thin gray colored plastic and the insulation is thin as I described before. Rhizzle is right on that the insulation is wet on some of them close to the air box.

    I guess I am also surprised that the contractor did not even mention replacing them.

    So would it make more sense to get them re-wrapped? or to have them replaced completely?

    Thanks
    In a humid environment the dewpoint is often above the temperature of the duct, so you must rely on the vapor barrier to prevent condensation. And remember that wet insulation is no longer insulation. You need to have a vapor barrier separate humid air from the cool surface, I cannot see how you can avoid replacing these ducts.

    A consolation prize is you have an opportunity to get really good work on sealing the ends and possibly reduce some air leaks, be sure to remember to press for duct sealing in the process. Professionals are usually not eager to focus on the duct system unless installing a new one. I know there are exceptions but most seem to consider this the grunt work which is not the best part of their job. I say this as a homeowner of course.

    Best wishes -- Pstu

  13. #13
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    RoadRash,

    Send me an email. My address is in my profile. I will send you information on sweating ducts, causes and corrections in a reply. Including formulas. You might want to put your email address in your profile. Sending pre written stuff in an email saves retyping a lot, as Udarrell has pointed out.

    You can also read the article in the second link I posted. What I can send you is an expansion on that.
    Remember, Air Conditioning begins with AIR.

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