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Community and Q&A

Dampness in AC air handler closet

Nola Sweats | Posted in General Questions on

I’ve got a damp air handler “closet” that has a mildew smell.  I’m looking for a good solution to the moisture.  I think the source of the moisture is the air handler itself — it’s a cold surface in a warmish area of the house in a hot-humid climate (south Louisiana), and water condenses on the outside of the unit.  I had a condensate line leak recently, which has been repaired, and I realized then that the space is always muggy inside.

I can put insulating board on one or two faces of the air handler, but the top and two other sides are inaccessible and would still sweat.  The space is probably too small for a dehumidifier — about 3′ wide, 5′ deep, 6′ high.  If not for the smell, I’d just add a small supply air or put a passive vent in the door, but it’s too musty in there.  Venting to the outside is possible but not easy, and I think and exhaust vent in my climate would just bring in wet air to condense somewhere else.  Is something like Damp-Rid the only solution, or am I missing something?

FWIW, the “closet” is really just an unfinished space, with air handler, ductwork, exposed studs, etc.  It’s within the building envelope, surrounded by conditioned spaces on all sides.  Thanks!

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  1. Jon R | | #1

    I'd look into some of the smaller dehumidifiers and if one could somehow fit in there.

    1. Nola Sweats | | #2

      Thanks. That's an option, because it wouldn't be too difficult to get power to that location. Is
      there a specific brand and model that's reliable?

  2. Expert Member
    Kohta Ueno | | #3

    If the geometry is what I am picturing, this may be relevant:

    BSI-006: No Good Deed Shall Go Unpunished

    But a dehumidifier would directly solve the problem, as well as pull down humidity in the house if connected correctly.

    BSI-012: Balancing Act - Exhaust-Only Ventilation Does Not Work

    1. Nola Sweats | | #4

      Thanks, Kohta. Yes, the no-good-deed article is similar to what I have -- an a/c air handler in an unconditioned closet centered within the living space (not a garage), sweating when the unit is cold. Considering the mold/mildew smell, however, I want to avoid just adding supply air to that little closet, because that will just push the smell into the living space.

      It was your recent Beer & BS Show on dehumidification that convinced me I needed to up my dehu game. The consensus opinion was there places like South Louisiana almost require separate dehumidification in addition to air conditioning.

      So I actually got a dehumidifier. Not the platinum Santa Fe, but a respectable 50L LG model. My plan was to locate that in my open-cell foam conditioned attic, (1) to isolate the dehumidifier noise from living spaces; and (2) to ensure I don't get the open-cell-foam wet sheathing problem. I've already got a smallish a/c vent into the attic per Lstiburek's recommendations, but your BS+Beer show convinced me to add the dehumidifier since I probably need one in any case. (If putting the dehumidifier in the encapsulated attic is a bad idea, please speak now!)

      I think the LG dehumidifier would be too big for my little air handler closet. The oversized dehumidfication for that small space might benefit the whole house, but the dehumidifier puts off a lot of heat for a small space, and it's loud enough that it might be spousally unwelcome in the kitchen where the air handler is located. A smaller dehumidifier is an option if things like Damprid aren't adequate. Or should I try the 70-pint LG in there, since I've already got it? Thanks again!

  3. Nola Sweats | | #5

    If anyone has a small dehumidifier they'd recommend for a damp HVAC equipment closet, please do. The area is roughly 150-175 cubic feet. If anyone can vouch for a solution like Damprid, please do.

    The damp closet is roughly 3' x 5' x 6' rectangular solid. On closer examination, at the top of the closet, there are open areas that extend between the 2nd-floor joists to the exterior walls (which would account for some of the humidity), so figure 70 cubic feet to be dehumidified in those joist spaces, which is how a closet gets to over 150 cubic feet.

  4. Steve Knapp CZ 3A Georgia | | #6

    I would look for an Energy Star rated unit with an onboard condensate pump. It probably won’t run that much since the space is so small.

  5. Norman Bunn | | #7

    I just got a Media 50 pint Energy Star unit with a condensate pump at Sam's for $180. The unit works well, is reasonably quiet, and a steal for the price. The instructions are pretty, but pretty poor. However, the control panel is easy to understand, just know that the comfort setting is for much lower humidity (35-45%), and the continuous setting is for slightly higher humidity (45%-55%). Seems well made. Time will tell.

  6. ForPetessake | | #8

    It may also indicate problems with a clogged drain pipe. Make sure that is cleaned out once a month using shop vac.

  7. Walter Ahlgrim | | #9

    Let’s consider another possibility. Given the location it seems likely the ductwork goes into the attic and is likely to leak like a sieve blowing lot of air out of the house into the vented attic and on to the outdoors.

    If so all that cold dry air is being replaced with hot moist air from outdoors thru some gaps someplace in the house if that someplace is this closet that air is likely to condense water on every cold surface.

    Fixing the leaky ducts and caulking the floor and ceiling penetrations could not hurt.


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