Indoor Air Quality/Musty Smell in Air Ducts w/AC


I am ready to pull my hair out.  Live in Houston, Tx in a ranch 1970's remodelled home. New air ducts were installed in attic 7/2010.  New coil/drip pan in 11/2011. New gas furnace in attice 10/2012.  Ran heat for month of November.  A/C was off. All was well until I turned A/C on early December.  Strong musty smell came out of air ducts.  Ran air for awhile to see if it would get better...did not.

Called A/C guy.  He put in an "air scrubber" (Ozone machine in attice attached to pleninum.) Didn't work.  Had air ducts and coil cleaned, sanatized, deodorized.  Smell back in 24 hours.

Now, we are looking at replacing outside a/c condenser, new freon, drain tubing.

Any thoughts to what it is or I need to do? HELP! Thanks.

Hello Kathy,

What you have described may be what is called in the HVAC Industry as "dirty sock syndrome". It usually occurs after an evaporator coil has been replaced, and when the cooling has been turn on right after the heat has been on.

The problem usually seems to be limited to heat pumps, but not always. In most gas fired furnaces, the coil temperatures exceed 160F, a temperature that would kill most microbial life. And in heat pumps, the typical coil temperatures during heating cycles is @ 125, a temperature that seems ideal for the supposed microorganism to still thrive. Since you live in Houston, where it's very hot & very humid, during the winter there, many days you'll need heat when you wake up in the morning, but by afternoon the air conditioner may be on. It's this short cycling of hot & cold that provides the perfect environment for some germs to prosper...thus giving you that lovely odor that you now have.

Cleaning & sanitizing is basically useless...more times than none, you'll have to replace the evaporator coil. I don't know if your manufacture warranty will cover that or not, but it's worth looking into. If they do cover it, then it would be best to get an all aluminum coil that has been'll be covered in a black's a special treatment that prevents bacteria from forming and along with being all aluminum will prevent refrigeration leaks. You may have to pay the difference, but it'll be well worth the investment. In addition, having some UV lights as extra preventative measures may not hurt either.

Kathy, I hope that this helps your situation...please let me know if this helps put you on the right path or not. If you need further assistance, then please ask.  

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TJ Nichols


Hello, I'd be glad to answer your questions on IAQ & other related issues!


My approach is to treat the home as one system with individual working parts. Think (TEAMWORK). When one or more areas (PLAYERS) are failing or bad, it affects the whole system (TEAM) to operate less efficiently. I aim to address the problem, not just the symptoms, to offer better solution options.

High School Grad (06/1991)(VA) EPA licensed & certified in Types I & II Refrigeration (04/1997)(MD)

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Have served tens of thousands of customers over the course of my career

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