The AC supply grille in my daughter's room has dirty, crusty stuff on it. I think it's mold. What should I do?

Asked by Bob Whitson
Arlington, TX

Earlier this evening, my youngest daughter called me into her room, pointed up at her ceiling and asked, "What's that?” What I saw was disturbing. The AC supply grille was covered in crusty, dirty, and ugly-looking stuff. I think it has mold on it. Two years ago, we replaced our AC system with new variable-speed heat pumps with dehumidifiers so I'm a little perplexed. I went from room to room and it's the only supply grille in the house that looks like this. My instinct tells me to call the AC company that installed and maintains the system but I'm wondering what the Green Home Guide experts think?


Steve Saunders

Answered by Steve Saunders

Irving, TX

TexEnergy Solutions, Inc

August 26, 2010

It is a bit disturbing to find supply registers that are covered with crusty, dirty and ugly stuff. I am especially sensitive to this problem as both of my daughters have respiratory issues and we are particularly careful with their rooms. So I can appreciate your concern and sense of dismay.

Here are my first observations and thoughts as to potential causes (not in order of importance):

  • If we were bypassing the filter, I would expect to see it at more than one grille. The fact that the problem appears on only one grille probably points to a localized problem.
  • If your system has a dehumidifier (a great idea, but rarely seen in the real world), it may need to be checked to ensure that it is functioning properly.
  • The supply terminal box (boot) may not be sealed to the sheet rock and insulated properly.
  • The problem grille could also be the last on the run and the variable fan speed setting needs adjusting.
  • It would be reasonable to check the air filter and make sure it isn't plugged or restricted in some fashion. Sometimes, it might have debris (crusty, dirty, and ugly) like the supply grille does. If that were the case, we would go in a different direction for resolution.
  • There could be a leak in the supply duct -- probably close to the end of the duct run (right before it gets to your daughter’s room) and that could be the cause of hot, moist air.

What to think?  It would not surprise me if it was mold or (most likely) a mixture of dirt particles and mold.

  • This is not quite as bad as most people fear. The vast majority of household mold is benign. That means there is not a major -- or even minor -- health risk in the home. Unless there are very significant and specific respiratory issues among the residents -- possible if you have installed a dehumidifier -- I would move carefully but without a major sense of urgency.
  • Over the last 10 years or so, we have taken 100 or more samples (all different homes) of “dirty, crusty and ugly stuff” and sent it off to the laboratory for evaluation. When I am personally involved, I am almost always surprised. Every time I am sure it is mold, the tests come back dirt. When I am sure it is dirt, we find that there is mold involved. My point is that the appearance is not a clear sign.
  • Only once have we had a test come back with what the lab described as “toxic mold.” In that instance, there was only a single toxic mold spore … thus insufficient to threaten the health of normal humans.
  • However, this does not mean you should not exercise care in dealing with the substance. It just means that you are not likely to be in major danger if you take reasonable precautions.

Action Steps

My judgment says that the primary culprit is the “venturi” effect -- caused by a lack of a good seal between the air terminal device (boot) and the sheet rock.

You probably have a gap and as air moves through the boot and the grille, it pulls air from the attic or interstitial space (between floors in a two-story home). The attic air is much hotter and carries moisture in it. As that air crosses the cold supply register the water condenses on the grille. The water traps small dirt particles (dirty and crusty stuff) and mold spores start to grow.

  1. Remove and throw away the grille (do this yourself or hire a service company to do it).
  2. Check to see that the boot (the boot is the air duct terminal unit that the grille is connected to) is connected or sealed. If not, grab some caulk and seal it to the sheet rock. Confirm that the boot is properly insulated above the ceiling.
  3. While the grille is off, shine a light up into the ductwork to see if there is mold there. If there is, you have a systemic problem and need to call a professional for observation. If not, and there is a gap at the boot/sheet rock -- you have most likely found and solved the problem.
  4. Check to ensure that the filter is clean and there are no other obstructions to returning air flow.
  5. Buy and install (or have installed for you) a new supply grille.
  6. Likely these steps will eliminate the problem. If the problem returns, you need to have a professional come to the site to diagnose the issue.

If you wish to confirm the nature of the material on the supply grille, there are a variety of options. We usually collect a sample and mail or take to a local company who can give us a very fast analysis of the debris. The company name is Quest Micro Analytic and their phone number is 214-351-4441. I do not have a web address or email for them. But they have been fast, accurate and reasonably priced.

Here are a couple of practical ideas for collecting samples:

  1. Scrape a small amount of the debris into a sealable baggie.
  2. Take a large piece of clear tape and stick it to the debris (dirty and crusty stuff). The debris will easily stick to the tape which you then paste to a piece of wax paper and place in an envelope.
  3. Deliver (mail or in person) to the testing company.
  4. Results can come in a couple of weeks (or you can pay extra for next-day reporting).

These are not sophisticated methodologies but they are relatively quick, relatively easy, reasonably affordable and (usually -- if you are careful) not dangerous.

For more information:

Read Tammy Schwolsky's Q&A "My daughter has gone to college, so her room sits unused. Should we close it off and close the AC vents?"

Tagged In: home air quality, mold removal

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