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How Do I Treat Mold in an HVAC System?

Updated on July 18, 2022 by Joseph D. Nielson

Table of Contents

Young woman with a facemask and cleaning products looks up in astonishment at ceiling mold

Unless you’re a cheesemaker in the Alps, the sight of mold is generally cause for concern. Beyond being unsightly, these microscopic networks of spores can pose real hazards to a home’s residents, aggravating allergies, causing itchy outbreaks and – in some cases – causing serious physical health problems.

Usually, we can tackle these sporous invaders as they arise. We comb through reviews of the latest cleaning products to figure out the best plan of attack, and then target the visible blotches on our walls and porous surfaces. We research how to clean a HEPA filter so we can fortify our defenses. And we might even plug in a few air purifiers in strategic places.

But what about the mold we can’t necessarily see, the stuff lurking in the hollow ducts and complex machinery of our central home systems? How do we fight an invisible enemy?

That’s what we’re discussing today: Specifically, how can you treat mold in your HVAC system. Home residents rely on their HVAC systems for a steady stream of temperature-controlled air, which is why it’s essential to ensure the safety and cleanliness of your HVAC system.

In this post, let’s tackle the issue from all angles. We explain how mold enters an HVAC system, why mold buildup is a problem, how to spot an HVAC mold issue, and – finally – what steps you can take to free your home of these microscopic adversaries.

How Does Mold Enter an HVAC System?

To tackle mold, you have to think like mold. (Not that they think, exactly – but they do have preferences, needs, and predictable behavioral traits!) 

Mold loves nothing more than a moist environment of organic material. Moisture allows them to thrive and replicate, and organic material provides them a square meal upon which to grow. In the wild, mold grows in a naturally harmonious way, flourishing in damp soil and feeding on stuff like dead tree trunks. They’re nature’s efficient way of recycling organic matter, a welcome presence.

In the home, they aren’t so welcome. Our homes are chock full of organic materials: wood, drywall, carpeting, upholstery and insulation. They also like to adhere to the insides of ducts, especially warm, moist HVAC ducts. If your home’s moisture isn’t controlled, you may be unknowingly courting these industrious little spores.

A modern living room with a ceiling vent concealing moldy air ducts

Why Is Mold a Problem?

Let’s get one thing straight: You are never going to get rid of mold completely. Some mold is just a fact of life. However, mold becomes a problem when it proliferates and grows, presenting itself in visible clusters.

These mold colonies are a problem for a few reasons:

  • Molds produce allergens and irritants that can bother home residents. Mold can cause itchy eyes, eye irritation and allergic reactions.
  • Mold may also produce mycotoxins, a potentially toxic and dangerous substance. According to the World Health Organization, mycotoxins “have been linked to long-term effects on health, including the induction of cancers and immune deficiency”
  • Mold can eat away at organic material, lowering the functional lifespan of some home elements. Therefore, tackling mold should also be seen as a prolongation tactic to protect your residential investments.
  • Mold poses a serious risk to immunocompromised individuals like organ transplant receivers and cancer patients undergoing chemotherapy.

If anyone in your home has allergy sensitivities or a lowered immune system – or you simply don’t want to risk allowing mycotoxins in the home – tackle your mold issues.

In some cases, an air purifier for large rooms will do the trick. Just be sure to check out our article on how to position your air purifier correctly. But in other cases, a more hands-on approach is required. Case in point: Your HVAC system.

How to Tell if Your HVAC System Has a Mold Issue

When should you spring into action? Here are a few ways to tell if your HVAC system has a mold issue:

  • Your air quality monitor shows an elevated number, indicating a poor air quality reading.
  • You see visible mold clusters around the vents, fans, condensate pans, plenums and/or air handlers. Mold is often fuzzy (or slimy) with a black, grey, white, brown or green appearance. 
  • You or someone in your home is experiencing sudden allergies whose source is unclear.
  • The air in your home consistently feels thick, humid and “swampy.”

 If you notice one or more of the signs above, we recommend inspecting your HVAC system.

Mold Remediation: How to Treat Mold in Your HVAC System

Normally, we advocate for a DIY approach. In that spirit, we are listing steps below for treating HVAC mold by yourself.

However, mold is a serious problem; loosening it from its stationary position can cause spores to disperse in your home. Unless you are a) confident in your mold-fighting abilities, b) healthy, and c) armed with the correct personal protective equipment, this is one of those cases where you should consider calling in professionals.

There – that’s our short disclaimer! Do with it what you may. If you’re eager to treat mold by yourself, here’s how to do it.

Turn off Your HVAC System

Start by turning off your HVAC system. Aside from making the cleaning process safer, turning off your HVAC system prevents the airflow of mold spores as you clean. Remember, you will be loosening a lot of mold from its resting place. You don’t want your fan blowing those freed mold spores around your home!

Look for a switch located on or near the furnace to cut the power. You can test that it’s off by laying a thin sheet of paper (a tissue works well) over a vent. If the tissue doesn’t move, you’re all set.

Strap on Your PPE

Personal protective equipment (PPE) is critical when working with mold spores. At a bare minimum, we need you to wear a tight-fitting N95 (or similar) mask. Luckily, most people have masks lying around these days! You may also consider gloves and protective eyewear.

If there are other people in the house – family, roommates, etc. – kindly encourage them to take a short stroll around the neighborhood. If they insist on staying put (or are unable to leave the house), ensure that they are also fitted with an N95 mask.

As an added precaution, fire up the air purifier. How long does it take an air purifier to work? According to our research, you should run it for roughly 45 minutes.

Man in yellow coveralls and full PPE sprays mold disinfectant

Replace Filters, Moldy Duct Wrap and Other Porous Material

Remember when we said that mold loves organic material? Well, this applies to pretty much any porous material as well.

Inspect the porous elements of your HVAC system, like the filters and duct wrap. If they exhibit signs of mold, like fuzzy patches or slimy texture, replace them.

Dispose of any removed material in a thick, sealable bag. Take it out to the trash immediately.

Look for Localized Signs of Problem

Next, you can execute a spot check of the non-porous surfaces. This includes the ducts, plenums, condensate pans, fans and vents. If the spots are easy to reach, then you should have no problem skipping to the next step.

If you see (or sense) that the problem runs deep into the ducts, you may need to rent a fogger (more on that below).

Apply HVAC-Specific Disinfectant

Thankfully, they make cleaning products specifically for this task! Ask your local hardware store about EPA-approved HVAC disinfectant spray, or find disinfectant sprays online. Lysol, Microban, Concrobium and Sniper each make popular HVAC disinfectant products (though they may not specifically say it’s for HVAC systems on the packaging).  

Follow the manufacturer’s directions on the label. Often, this means allowing the cleaner to sit for a short period (around five minutes) before wiping it clean with a rag. We recommend using a rag that you don’t care too much about, since you should discard the rag (in a thick, sealable bag) after you finish.

Rent a Mold Removal Fogger for Air Duct Cleaning

The next level up in mold remediation is to bring in some “firepower” in the form of a mold removal fogger. Mold removal foggers look like miniature canons, and they produce an antimicrobial mist designed to kill mold in large or inaccessible spaces.

These tools can cost upwards of $600, so they aren’t cheap. For that reason, we usually recommend renting a mold removal fogger. The exception is if you are a landlord or building manager, and foresee needing a fogger for multiple properties.

Follow the instructions included with the fogger – to a tee. As with any chemical dispersal system, proper ventilation, safety, and HEPA-filter purification are essential. Our reviews of the latest humidifier and purifier models can help you find the HEPA-filter, but you will need to follow safety instructions as well.

Use Mold Inhibitor Sprays to Prevent Reoccurrence

After you have removed the visible mold, whether by hand or with a fogger, you want to make sure it never gains a foothold again. To prevent reoccurrence, spray all components of the HVAC system with an “inhibitor spray,” which creates an inhospitable environment for regrowth.

Concrobium, RMR, Home Armor and Mold Armor each make popular inhibitor sprays that work very well.

Find NADCA-Certified Cleaners for Reliable Air Quality Management

Mold is sometimes classified by its extent: small, medium or large mold extents. According to government guidelines, large mold extents require professional cleaning.

If your home’s HVAC mold is extensive, or you simply don’t want to deal with it yourself, leave the mold-fighting to NADCA-certified HVAC cleaners. (NADCA stands for National Air Duct Cleaning Association).

Provide them with pictures of mold in air ducts or allow them to inspect your home. They will advise on the best mold control for HVAC systems and air ducts in your home, and then present you with a job quote. How much does it cost to remove mold from air ducts and HVAC systems? Anywhere from $600 to $2,000. The price is steep, but the results will help you – literally and figuratively – breathe easier in your home.

It’s time to “break the mold” and start fresh. Inspect your HVAC system for mold and take proactive steps toward clearing harmful mold for a safer, fresher home.

About Joseph D. Nielson

Former journalist and editor for various press groups, I now dedicate my time to reviewing products for the home and family life. When I get time to myself, I enjoy rock climbing, taking my dirt bike for a rip, and most importantly providing my family with the best home possible!