Wet ceiling arond AC ' ducts

Discussion in 'HVAC Heating & Cooling' started by flazoom, Jun 23, 2009.

  1. flazoom

    flazoom New Member

    Messages:
    22
    Location:
    South Florida
    Hello,
    I live in south Florida and run my AC pretty much year round and have a question about the ceiling AC ducts. All my ceilings around the ducts are wet to the point the paint has bubbled a bit. There is no dripping but the ceiling drywall is quite damp when I removed it. It is only around the vent. the AC ducts themselves are square where they feed the rooms and seem to be made of metal foil wrapped fiberglass board (I apologize for not knowing the correct terminology).
    My thoughts are the metal 'louvered vents' that attach to the ducts may be getting condensation that is then absorbed by the drywall. Does this make sense and how can I stop it?
    Thanks in advance.
  2. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    Many things can cause this, the one that pops in my mind first is an oversized system. What are some particulars about your house - square footage and unit size? Is your duct system sound - regarding rips or tears?

    I'm in SW Florida - no sweating on my system or anyone else that I know.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    To amplify 'too large'. Ideally, a system runs all the time. This makes the air go over the evaporator coil many times, each pass removes moisture, eventually drying the air. If the system is oversized, it runs a short time, then shuts off. The ducts are now cold, and the air may be, but since it didn't run through the evaporator coils multiple times, it is now cold but clammy in the house.

    Or, you could have lots of air leaks, with no vapor barrier in the walls, and it's just sucking in moisture. Just like a cold glass, anything below the dew point will have condensation on it. The key is to remove enough moisture to lower the dew point of the house air temp and keep it there.

    If you only run the a/c part of the day, then open the house say at night, you will probably never get it dried out.
  4. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    When I was learning basics I made that comment to my Father in Law, he about had a heart attack imagining the electric bill. :cool:
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Another thing that helps is a variable speed air handler. Trane had a comment about identical houses with the same air conditioner. One had a variable speed version, the other a fixed speed one. Over the season (it must have been long!), the variable speed version pulled enough more moisture out of the air to fill a backyard swimming pool. By initially running the fan slowly, each pass across the evaporator pulled more moisture out. Then, only if it couldn't meet demand within a reasonable time, did it ramp the speed up. A nice side effect is that you never really hear the fan start or stop since it ramps up and down rather than banging on (really annoying if there's any flexing ductwork).
  6. flazoom

    flazoom New Member

    Messages:
    22
    Location:
    South Florida
    Thak you for the responses. My house is around 2000 sq feet (1,934 to be exact) under air. The A/C is a 13 seer unit. Is this the 'size' information? (I don't know too much about HVAC as you can see)
    The house was built in 1993, the interior air handler is the original (RUUD) and the outside unit is new this year (It is a Goodman if that matters and has 13 seer lable). I need to get up in my attic to make sure the duct work is solid. Is this size unit ok?
    The moisture problem is on the second story vents in 2 bathrooms and the master bedroom. The 2 other bedrooms seem to be ok and there is no problem with downstairs vents. I thought it might be underpowered fans in the bathrooms, but that does not explain the master bedroom.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    SEER is a measure of efficiency, not capacity. It will be labeled something like 2T or tons. That is the capacity. A ton of a/c = 12K BTU/hour, if I remember properly.

    Without running a heat loss analysis on the house, there is no way to tell if it is the right size. This would take into account the outside temps, desired inside temps, the house orientation, size, insulation, window and placement and other things.

    Empirically, if the a/c compressor only runs for a very short time, it is probably over-sized. Lots of short on/off cycles is very inefficient, and is lousy at removing moisture from the air and longevity of the compressor.
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