Mold In HVAC System

Signs of Mold in Your Heating & Air Conditioning System

You only need to do something about mold in your air conditioning system if there are obvious signs, such as visible mold or a strong mildew smell.  The typical places you want to check for mold include the air ducts, intake vents, cooling coils, and drip pans.

If you can’t see any signs of mold, but there is a definite odor, you should call a professional to inspect your system.  Mold in your air ducts or other parts of your air conditioning system should be dealt with immediately.  Failure to address this problem can encourage mold spores to be spread throughout your house as the system blows air across the mold and into the various parts of the house.  Do not run your air conditioning system, especially if mold is present in your air ducts or near the intake vent.

Causes of Mold in Your Heating and Air Conditioning System

The two major causes of mold in your air conditioning system are condensation and organic material.  Condensation occurs when parts of your air conditioning system get cold, especially when the surrounding air in the room is warm and humid.  For example, if your basement tends to be humid and the air ducts get cold while the air is blowing, the water vapor in the room can begin to condense on the air duct, creating a small amount of water.  If the humidity is low enough, the water will quickly evaporate once the a/c turns off.  If the humidity is too high, then the water will persist and that’s when you begin to have a mold problem.


Air conditioning systems tend to collect dust, both in the unit itself and within the air ducts.  Dust contains all kinds of organic material, such as pollen and dead skin cells, which is essentially food for mold.  Combined with a damp environment, this creates a great place for mold to grow.

Heater and Air Conditioner Mold Removal

Removing mold in your air conditioning system can be very difficult because a lot of places are hard to access, especially areas inside your air ducts.  Additionally, mold in your a/c system can have serious consequences since it easily spreads mold spores throughout the entire house.  For these reasons, we recommend you call a professional mold removal contractor to tackle this job unless the mold is in an isolated area and it is easy to access.  For tips on best ways to select a good contractor, check out our Hiring a Pro section.

For removing small areas of mold, try the following:

Step 1- Choose a cleaning solution

  • Mixture of household detergent and water (recommended by the EPA).
  • Commercial mold removal product (always follow manufacturer’s instructions on the label).
  • Baking Soda -Detergent Solution (1/2 cup baking soda, 1 cup water, 1 Tbsp mild liquid detergent).
  • Borax Solution (1 gallon of water to 1 cup of borax, or 1 part borax to 16 parts water).

Special note on Bleach:  Bleach is not needed on hard, non-pourous surfaces, such as sheet metal.  One of the options above will be adequate, and will also be a lot more safe.

Step 2 – Put on Protective Clothing and Make Safety Precautions

Depending on the severity of the cleaning solution that you chose, you will need to take some safety measures in order to keep yourself free from harm (see our protective gear page for more detail).  Because you are working with mold, we recommend the following:

  • A respirator, or air mask, that is adequate for blocking mold spores from entering your lungs.  The EPA recommends a N95 mask or equivalent.
  • Rubber or Nitrile gloves.
  • Safety goggles that do not have air vents in the sides.
  • If you are moderately sensitive to mold exposure, we recommend wearing coveralls to protect your skin as much as possible.  If you are severely sensitive to mold exposure, we recommend getting somebody else to perform the task.

If you are using a product that has strong or dangerous fumes, also make sure your are working in a well ventilated room.

To see more on how to prepare to remove mold, click here.

Step 3 – Apply the Cleaning Solution and Scrub

Apply the cleaning solution that you have elected to use.  You can do this with a spray bottle, a lightly damp rag, or a low-abrasive brush or pad.  Do not use anything abrasive, such as a wire brush.

Apply the mold removal solution generously. Let the solution sit for a few minutes, then scrub the area in circular motion with your rag, brush, or a scrub pad.  Using a disposable towel, or a regular towel that you can disinfect with bleach later, wipe off the area and the excess.

Continue this process until the mold is removed from the surface.

Step 5 – Clean Up and Let Dry

After the mold is removed, clean up the area and either dispose of anything that has had contact with the mold, or clean it with a proper detergent or fungicide.

Let the area dry by keeping it warm and with good ventilation.  If you live in an area with higher humidity, you may want to run a dehumidifier.

Step 6 – Check for Signs of Mold

If mold begins to reappear in the next few weeks, it is probably because of condensation.  Look for ways to improve air circulation, perhaps by opening a window or running a fan or dehumidifier.  If this isn’t possible, try to insulate the area that tends to stay wet.  This will help reduce the temperature difference between the ambient air, and the effected area.  This will disrupt the mold growth process and prevent it from getting established.  Remember, reducing moisture is the probably the number one factor in air conditioner mold removal.

If the mold reappears, repeat steps 1 through 5.

Prevent Re-Growth of Mold in Your Heating and Air Conditioning System

Depending on the situation and where the mold tends to grow, keep in mind the following principles of mold prevention in your heating and air conditioning system:

  • Change the air filter regularly, typically every 2-3 months.  Filters capture fine particles, which can create a place for mold to grow.
  • Run a dehumidifier.  This is helpful, especially in areas that tend to be damp and cool.
  • Insulate if possible.  By insulating air ducts, especially in a damp place like crawl spaces, the air immediately around the duct will be kept at a similar temperature and will reduce the amount of condensation.  This works for cooling coils also.
  • Hire a profesional to clean the HVAC system, including the air ducts.  Only perform this if you know your air ducts are cluttered with dust and debris, otherwise you could be just wasting your time and money.

We hope this was helpful!  Please feel free to leave a comment or question below.  If you would like to see how other people are handling there air conditioner mold removal issues, try going to the Mold Blog’s Q&A section.

17 thoughts on “Mold In HVAC System

  1. This is a great blog post, really gives detailed steps on the right ways to handle this problem. If you need any extra tips on how to test and inspect your air ducts check out this blog post from A Accredited Mold Inspection Service, Inc. They have great information on their site that can definitely help you make this daunting task easy and affective

  2. If you have mold in your HVAC system, chances are you probably need to check and clean your air ducts. For more information on removing and cleaning mold from your ducts check out . I had a problem with mold last year and they were definitely the answer to all my prayers. Even if you aren’t in Pa, you can learn a lot from their website.

  3. How do you know that the smell coming out of your vents is mold rather than just the A/C starting up? I know I’ve sometimes smelled a sort of musty smell when the A/C starts up from time to time. Are there other ways to tell you have mold in your system?

  4. bravo!you give me another point of view about air conditioner..thx a help me so much!this is the article that i am looking for!

  5. thank you so much!your article about air conditioner is very gives me lots of information.i have book marked your website!keep writing.god bless you..

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  7. can the plastic movable parts at the front of the AC be removed? They were heavily encrusted with mold. I unplugged the AC and sprayed with a bleach solution spray I bought and repeated a number of times till it was all gone but can still see mold behind there which the spray doesn’t reach. I don’t dare try to pry out the movable plastic sections, two rollers that can be adjusted to direct air flow.

  8. What about mildew smell (new construction home as of 6/30/15? This started on 5/17/16. It is in a hall bathroom and when I open the cabinet under sink that smells. This bathroom is hardly ever used too. There are no leaks nor in the attic. Nothing dirty. Ducts and coils checked.

  9. Beverly, Bleach can severely corrode the metal parts of an air conditioner. Most of the plastic parts can be removed. However, it is often a challenging puzzle as to the order of removal.

    Jenny, The trap in the sink and tub/shower may be dry. Sewer gas will enter the room through the empty drains.

    Also, even a tiny leak under an enclosed sink can cause a stinky odor. Put a paper towel under the sink and check it once a day for moisture.

  10. I have a new AC unit and even cleaned the attic, but am still getting sick from the AC, I wake up and can’t breathe from lung problems. Somebody please help!!!

  11. Thank you for your attention to detail and great writing style. Your professionalism shows in your article. I like your interesting views and appreciate your unique ideas. This is quality.

  12. @samuel Friedman there might be mold in your duct work. (The pipes that bring the air into your rooms in your house. You should get someone to come in and check them out or you can go in and try to clean them yourself. I cleaned my own ducts and treated them with a mold inhibitor that I purchased from Home Depot. It might be difficult to clean your ducts out depending on location, etc.. Also make sure you are using a high quality AC filter that will keep out mold and other contaminants that can cause breathing problems.

  13. We had all duct work replaced due to mold/ mildew on some of the registers. Also replaced central air because of crack in it. Since then we have a much worse mold/mildew problem now on every register and lots of humidity in area of are intake. Any suggestions on how to clear this problem would be greatly appreciated.

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