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How to Spot a Potential Mold Problem in Your HVAC System

by Catherine

HVAC units are often the most overlooked sources of mold and mycotoxins in a home. They are also the most challenging for mold sufferers, because they distribute (literally by blowing) the mold spores throughout the home. Sometimes, there is obvious water intrusion, or damage somewhere to the home, and water and mold may be obvious threats. When this is the situation, in most cases, the issue can be fixed, the space remediated, and hopefully, the problem halted and solved in one fell swoop. While this sort of mold issue does have its own set of challenges, mold inside of an HVAC system can be insidious and difficult to remedy. I know, because this is exactly what happened to my family. We purchased a new-to-us home, had all of the inspections and code check-ups required for us to make thoughtful judgments on our purchase, and we still were injured by hidden toxic mold in our HVAC system that cost us quite dearly in terms of our health and our finances.

Now, I realize that you cannot live in fear and that professional inspectors are hired, so that you can trust their analysis, but sometimes, especially if you are someone who is aware of the health repercussions of living in a mold-contaminated environment, it is imperative to do your own reconnaissance. To that effect, I want to share with you my tips and advice on inspecting and testing an HVAC system for mold “DIY style.” I am obviously not a HVAC professional, so none of this advice is technical in nature. What I am is a mold survivor. As such, and as someone who now knows what to look for from personal experience, I thought this post would be particularly helpful to others out there, who are also concerned about their health.

Before I begin my list, I want to tell you that this list is not directed just at someone looking to purchase a home. This list is meant for anyone and everyone, even if you are currently living in a home, where there is no obvious leaking, water intrusion, ventilation issues, or visible mold. It is just DIY knowledge that will help you to better understand mold, where it can hide in an HVAC system and how you can be vigilant about it to better your health. That being said, here it is:

Tips and Advice, AKA, The DIY Version of Inspecting Your HVAC System for Mold

*If you are chronically sick in your home environment, and can find no visible mold, no leaks, or water intrusion, but feel that your home is somehow making you sick, please, please inspect or hire a professional to inspect your HVAC system for mold—especially, if your symptoms noticeably  improve when you are away from your home. Take into account that if you go through this list and find mold at any of these suggested checkpoints, immediately contact an HVAC or indoor air quality professional and/or a mold remediation professional. A professional HVAC person can open up the system to look at the coils for mold and dampness.  Not all HVAC professionals understand mold and lack certification. A pro who does can clean the inside with special solutions to clean the mold.

  1. Walk around your home and do a visible mold inspection of your supply vents. This means exactly what it says. Walk around, looking for any visible mold discoloring the vents. If you want to be very thorough, grab a screw driver and step stool, and remove a few of your accessible supply vents. Make sure to wear a mask when doing this. Flip the vent screens over and take a look. If there are mold colonies (small dark spots are actually very large mold colonies containing millions of reproduced microscopic mold spores or fungi) visibly growing on your supplies, it is a high probability that there is a mold issue in the system. In order for mold to grow, there has to be excess moisture and condensation that is lingering and not being dried out. It could mean a problem with the HVAC system, like water pooling inside, which can be later diagnosed by a professional, or it could just mean that your home needs some additional returns or ventilation solutions installed to move the air more efficiently. The most important takeaway–if you see mold on your supply vents, all of the air blowing throughout your home is coming through moldy vents. This is not good or healthy and needs to be fixed.

    A nice, clean supply vent is a good sign that air quality is stable.

    This is a dirty, moldy supply. Not what you want to see.

  2. Do a visible inspection of the exterior of the HVAC system. Look at all of the connections, joints, etc. You DO NOT want to see any visible mold growing on the exterior of the unit. You also want to make sure that there is no condensation forming on the outside of the unit while it is working. If condensation is forming, it is not sealed and working correctly, and chances are, it also has mold growing on the inside too.
    These are all pictures from just the exterior of our toxic HVAC unit.
  3. Check to see if the HVAC system has a humidification system attached to it. Some folks, especially those without a proper understanding of mold, think that their indoor allergies are caused by dryness. To remedy this, humidifiers are sometimes installed on the HVAC unit to provide a constant source of moisture to heated air as it pumps throughout the home during the wintertime. This additional moisture is being pumped inside your system and your vents to linger inside your home. In terms of mold, this is NOT good. All of this excess moisture in the HVAC system and in your air promotes the growth of both mold and bacteria. If your system has one of these, it should be turned off and removed. It could be a health hazard.

    Here is a picture of a humidification system attached to the HVAC central unit. You can see the water line going to it on the side of the unit. It is the box with the red label on it.

  4. Check to see if the HVAC unit has been set up to drain properly. Grab a ruler or a level. First use it to check that the actual unit is level. This means that it should not be tilted or on a slope. If this is the case, it needs to be fixed, because the water probably cannot exit the unit correctly. Many HVAC systems will have a water pump that will drain via a tube to the outside the house through a wall. Make sure that it is working. Then, use your ruler or level to see that the pan that the HVAC unit sits in is level. If the pan slopes, make sure that the tilt is toward the overflow drain. If the pan is tilted away from the overflow, chances are, water can pool in the pan, overflow and cause a problem.  If water is escaping or overflowing from the pan, it is a problem to be addressed by a professional.

    This was a hard picture to take to show how level the pan and drain are. Here you can see the bottom of the HVAC unit in a perfectly level pan with the drain at the side of the pan.

  5. Check the overflow pan that HVAC unit sits in for water and mold. This is another easy way to check to see if it is draining properly. If the pan is full of water, and there is mold in the pan, you need to get a professional to come help solve the problem, whatever that may be.

    Here, you can see our level, dry and clean overflow pan that the HVAC unit sits in. This is what you want it to look like.

  6. Do mold plate testing on the air coming directly out of your vents. Pick a few rooms in your home and perform mold tests on the air with testing plates. I use the EC3 Mold Screening Test Plates. This is an easy test to do. Simply purchase the test plates, and perform the air test per package instructions on the air blowing directly from the supply. If any of your tests culture more than 3 colonies of mold, you definitely need to call a professional out to your home to inspect the HVAC units specifically. (Note: Mold plate testing can easily be done at any step in this list. Never underestimate the value of actually testing to know what is going on. The EC3 Mold Screening Test Plates are invaluable tools for this sort of thing.)
  7. Check intake and HVAC unit filters for mold. The filters will definitely contain dust, especially if they have not been changed in a while, but they should NOT have visible mold growing on them. If they do, that means that moisture is being trapped in them with the dust and causing mold to grow. That is either an indication of excess humidity in the home that needs to be fixed or excess moisture in the system itself, which also needs to be fixed. If you check the filters and see mold, call a professional to help you figure out the problem. You can also cut out a small piece of the filter and send it into a lab to test for mold and mycotoxin presence.8. Check ducts for fiberglass. Take a good flashlight and shine it inside the supply. If you see any fabric or fibrous material, the ducts are fiberglass and will likey need to be replaced. Metal, smooth ductwork is by far the best, in terms of mold prevention and overall health. Smooth ducts can be easily cleaned, and wiped down and do not trap dust and moisture—food for mold.

    You do not want to look inside the vent and see this type of duct. Want hard, smooth, metal ducts that do not contain fiberglass.

    Here, you can see inside my clean supply vent to the metal duct that the air flows through.

9.  Listen to how often your air conditioner cycles on and off.  A properly functioning HVAC system will cycle on and off and will do an adequate job moving air and keeping humidity down. A dysfunctional system may cycle on and off less often, maintaining  the temp within a “band” of temperatures. A home that has sufficient air flow maintains a consistent temperature with less obvious variability.

10.  Check the smell of the home and how you feel while inside. If the air smells musty, moldy, stuffy, or like dirt, there is probably mold in the home somewhere or in the air supply. A home should NOT smell this way. Also, and this, in my opinion, is the most important test, check in with your body. If you feel sick, unsteady, fatigued, dizzy, anything different from how you normally feel when you are inside your home or a home you are contemplating purchasing or renting, do not ignore that. That test has never let me down and has saved me from entering or living in buildings or homes that could’ve made me and my family very sick.

Here are pictures of our beautiful, new HVAC systems.
We have one for the upstairs and one for the downstairs of our home.

I hope this list helps and empowers you to become a mold sleuth and to never stop doing the little things, like inspecting an HVAC system, that will keep you and your family safe from mold.

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1 comment

Micro Clean DFW June 1, 2022 - 7:15 am

I’ve been a heating and cooling contractor for over ten years, so I’ve seen just about everything regarding HVAC system maintenance. One of the most common problems I see is mold growing in air conditioning or heating supply vents.

There are a few ways to spot a potential mold problem in your HVAC system:

-Check the ductwork: Make sure there are no tears, rips, or other signs of damage anywhere in the ductwork. Also, ensure the insulation is intact, and there is no bubbling or peeling. If you see any damage, consider having it inspected by a professional.

-Check the insulation: If there is any evidence of moisture infiltration (i.e., water spots on the insulation), then there is likely a mold problem.


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