Black stains on air handler supply duct

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I noticed these black stains on my air handler, kind of looks like the joint isn't sealed real well and condensation forms and runs down the side of the unit, hence the white streaks on the side of the unit. So I'm wondering if the solution is to just apply more mastic over this to beef up the seal, or remove and replace, or something else. Also, curious why the color is black, is that soot mixing with the condensation, or something worse? Thanks.
q black stains on air handler supply duct, home maintenance repairs, hvac, plumbing
  12 answers
  • Marianna Martyniuk Marianna Martyniuk on Feb 19, 2015
    It looks like mold/mould you need a special cleaner for that. Black mould is particularly difficult to shift.
  • Nikki Smith Nikki Smith on Feb 19, 2015
    You can kill black mold with bleach. But you should wear a mask so you dont breath in any spores. There are many other household & natural cleaners you can use as well
  • Mcgypsy9 Mcgypsy9 on Feb 19, 2015
    Sorry I would definitely call a specialist in for this one! Black mold is nothing to deal with on your own! There might me more in other areas of the home as well.
  • William Tillis William Tillis on Feb 19, 2015
    Cleaning up dangerous molds can get expensive. There are short and long term health issues depending on the type of mold you have.Know what type you are dealing with would by my first step. Your county health dept should be able to direct you to people that can test your mold. I have researched mold before and you would be wise to know what the health indications are. Know what you are doing and what you are dealing with before you make any attempt at cleaning,etc.
  • Robin Robin on Feb 19, 2015
    Looks like mold, You should clean it with a bleach mix. The wrap your plenum with insulation. I am guessing that it has insulation inside ,but the top is missing from when the take off was cut in.
  • Nancy Nancy on Feb 19, 2015
    It looks more like mildew to me--like you would get in your laundry if you left wet clothes in your washer. Bleach would take care of mildew, so try cleaning it with bleach before you re-do any joints. There may be some inside the plenum as well, so open it up before you seal again.
  • Barbara Votik Barbara Votik on Feb 19, 2015
    Call a duct cleaner because what's inside is coming outside. You will breathe better too. There are several methods of duct cleaning. I prefer the one where a brush is used rather than only a vacuum. If a vacuum would work, why not put a big one at the front door! Once ducts are cleaned, clean ceiling and vent. My family once owned a cleaning business and we did very well cleaning ducts.
  • Pat Nolan Pat Nolan on Feb 19, 2015
    Its mold, not soot. The stuff can live on condensation and dust from the air. I would use a bottle of tub and tile cleaner or bleach and wipe it away. I would also not freak out if thats all the mold that you have. If you have clumps of it growing on the insides of your walls, then you have a problem with humidity. Unless you are seriously allergic to molds, don't panic. Better still, just don't panic until we tell you to.
  • D & K D & K on Feb 19, 2015
    There are several ways to effectively clean and remove mold and many of these have been expressed previously. The problem however, is not it's removal but what to do to prevent it from happening again. Mold likes moisture. Without it mold cannot live. So what happens when you have a chilled surface created by your AC system in contact with warm moist air...condensation and future mold growth. Regardless of which method you choose to remove the existing mold, insulate and seal the entire exterior of the air handler. When the moist air can no longer get to the chilled surface of the air handler, no more condensation will form and no more mold. This is the primary reason AC ductwork is insulated. Not to keep the air inside the duct warm or cool but rather to prevent condensation from forming from the differences in temperature and humidity, allowing mold to grow and spreading it throughout the house every time the blower engages.
  • Debbie Harris Debbie Harris on Feb 20, 2015
    Dennis is right! Fix the problem then have the ducts cleaned.
  • Julie Julie on Feb 20, 2015
    Cleaning the mold is only the short term fix. You need to ask yourself why is there mold at the plenum? What is the root cause of the mold? This is a sign of a bigger problem. You need to have a mold count done in your home & have the whole house professionally cleaned. Some people with compromised lung function or immune systems can become quite ill if they are sensitive to mold spores. If you try to clean it yourself the spores may dislodge, become air borne and contaminate every surface in your home. Be safe.
  • Alton Alton on Feb 21, 2015
    Looks like condensation has run down the side before . Reinsolate the top and install a UV light will kill spores and smells coming from duct work . Not cheap but worth it .
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