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  1. #1
    Join Date
    Jan 2009
    Posts
    285

    AC Plenum sweating/rust

    I have noticed in my attic the galvanized 'stuff' seems to be rusting. Today i was up there looking at another problem and noticed the plenum..or whatever it is the duct lines come out of, was sweating...a lot. It runs off the side, onto some of the flexible duct and then drips onto the attic floor, causing it to stay damp and its black with mold and mildew. I have scheduled service, but what is the acceptable repair for this? This system is less than 10 years old, and there are parts that are 'crunchy' with rust underneath the mastic. Its a heat pump unit with a variable speed compressor and handler. It runs...a LOT. IM in texas, and the unit outside usually doesnt spend more than 1 minute off, unless its night time. Its usually on the low setting, but between the zones, it runs a bunch.

    Any ideas on the sweating? Aside from that the system cools fantastic.

    Thanks.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,591
    Have them reseal it. And add insulation to it. If its not rusted through.
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  3. #3
    Join Date
    Jan 2009
    Posts
    285
    OK. Yeah, i went up in the attic yesterday and thought "Wow...its HOT up here", then when i got near the AC unit the temperature was noticeably cooler.

  4. #4
    Join Date
    Jan 2009
    Posts
    285
    This ac stuff is in a part of the upstairs that is unfinished. In other words, where its at is going to be the attic and the other side is going to be a room. If i were to wall out the area with the AC stuff in it and run air conditioning to it when the upstairs is finished, that would help with the sweating right? Also, when that is done, does that duct run off its own thermostat upstairs or does it run off of a thermostat in another zone in teh house? If this is qualifies as a DIY question, then please disregard.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,591
    If you made it a finnished insulated mechanical room. Yes, you could condition it, and eliminate the sweating that way.
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  6. #6
    Join Date
    Jan 2009
    Posts
    285
    Would it require its own 'zone' and thermostat? Im going to have to have them out to do the install of the second system for the upstairs, so I could add it then if thats how its supposed to be done.

  7. #7
    Join Date
    Jan 2009
    Posts
    285
    One thing i dont understand abotu the sweating issue is that i calculated my dew point to be around 70 degrees last night when i check it. Now, the air coming out of my vents is colder then that. Thats cool...no problem, but here in texas, AC units are put in the attic all the time, and i have never seen one that has the air handler 'insulated'. Do these just sweat and soak into the plywood subfloor in teh attic? If this is how it works, then mine has been sweating for 8 years or so, as evidenced by the warped plywood and black mildew growing there. Why arent these air handlers insulated on the inside? I ask because there is no way to insulate this unit where it sits, just not enough room to get all teh way around.

    In the summer here the attic can easily get to 115 degrees and the humidity outside is almost never lower than 35%, meaning the dew point is in teh 80's. There's no way this unit can 'not sweat' in these conditions. So, what im asking is, if this is the case, how in teh hell can you get away with installing a unit and not insulating it?

  8. #8
    Join Date
    Apr 2009
    Location
    Pan Handle, Fl
    Posts
    600
    Air handlers are insulated on the inside, if the insulation is falling apart or fell off, it needs to be reinsulated or glued back on. Having it in a conditioned space is the best way to prevent the sweating and molding. I think some manufacturers have insulation kits for air handlers that are installed in unconditioned spaces like attics and garages. We have a lot of units installed in attics and garages here in the Panhandle. I don't think I have seen one yet that didn't sweat at least a little with the heat and humidity we have here.

  9. #9
    Join Date
    Jan 2009
    Posts
    285
    Mine is a lenox unit, and the galvanized areas just after the box filter are all cold...like AC cold, all teh way around. That leads me to believe its not insulated....maybe my terminology is all wrong, maybe thats not the air handler. I thought it was just a plenum....and from there I have several large ducts that go to smaller boxes which branched duct work goes to each zone.

  10. #10
    Join Date
    Jan 2009
    Posts
    285
    Of course the AC guys cant get out here since im sure they are swamped with units that dont work, so i went up to try to put mastic on it. THe mastic takes some time to dry and since my house heats up about 1 degree for every hour the AC is not on (since its 100) outside, I cant leave the AC off long enough to get it to dry. It sweats so much that the mastic cannot dry. I also found a couple of ducts that leak enough air for it to be 'cool' up there around the unit. This system is less than 10 years old. I guess i need to get them out to come up with a solution to insulate the ducts or something because it is even damp between the insulated ducts, and of course beneath the plenum its black with mold and the floor is warped. I guess this is normal, but the plenum is covered in surface rust, and at the joints its nearly eaten through. I put the mastic on yesterday, so the unit doesnt stop sweating or stop running long enough over the night to let it dry.

  11. #11
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    you say the system is ten years old but are the ducts ten years also or older.

    if it is internaly lined the liner may have fallen off and you may need to replace the ducts with rapped ductwork

  12. #12
    Join Date
    Jan 2009
    Posts
    285
    The ducts are all the same age as the equipment. The part that is sweating is the plenum, off of this plenum are 4 LARGE ducts. One for each of the three zones and a 'wild zone'. ITs sweating most at the point closest to the air handler. Everything past that is dry. I may just put a drip pan under it.

  13. #13
    Join Date
    Feb 2008
    Location
    Waffleville
    Posts
    10,339
    Quote Originally Posted by beenthere View Post
    If you made it a finnished insulated mechanical room. Yes, you could condition it, and eliminate the sweating that way.
    is an excellent solution to the challenge
    If Guns Kill People, Do Pencils Misspell Words?

    http://www.youtube.com/watch?v=An2a1...eature=related

    Before we work on artificial intelligence why don't we do something about natural stupidity?

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