Your Guide To HVAC Mold & Mold In Air Ducts

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Bathroom mold, basement mold, shower mold, and even attic mold are usually what comes to mind when you ask someone to think about areas of their home that are prone to mold. But did you know that almost every HVAC system develops mold at some point? Even more frightening is the fact that the fastest way to spread mold through a building is through a forced-air HVAC system.

Every time your HVAC system turns on, thousands of microscopic mold spores travel through the air ducts of your home and circulate through the air, eventually finding a cozy spot to land and potentially flourish. The occupants and visitors of the home are also breathing in all these flying mold spores which can cause a host of health issues.

What should you do? How do you deal with HVAC mold and mold in air ducts?

Never fear! I am here with…

Your Guide To HVAC Mold & Mold In Air Ducts

Today we will take a look at a wide variety of HVAC mold related topics so feel free to use the table of contents below to skip ahead to any section. Fair warning – you might miss a really important nugget of info about AC mold.

HVAC mold and mold in air vent

Why Does Mold Grow In An HVAC System and Air Ducts?

This is a very simple answer. Mold needs moisture and food. There is an abundance of both in your AC unit and your home’s air ducts.

The moisture comes from the condensation present in air ducts. When your home is warm, water vapors can form and collect in air ducts as cold air makes its way through them. If high levels of humidity are present, the water will remain instead of evaporating which allows mold to grow quickly and en masse.

With enough water present in the air ducts, outside particles such as dust and debris can settle in the water. The combination of organic and inorganic materials provides mold with the food source it needs to grow, multiply, and travel throughout the duct system.

Signs Of Mold In Your HVAC or Ductwork

  • Strong smell or musty odor which is more pronounced when your air conditioner is running
  • Visible mold around air ducts or intake vents
  • Noticeable leaks or puddles around HVAC system
  • Blocked drip pan

Because the signs of mold are not always easy to spot, it is important to know when you need to bring in a certified mold inspector to conduct additional mold testing. Basically, if you think there might be a mold issue but you cannot confirm it with a visual, call the pros.

Common Causes Of Mold In Your HVAC System and Parts Of Your HVAC That Are At Risk For Mold Growth

The most common parts of your HVAC that are at risk for mold growth are:

  • Air ducts
  • Intake vents / air vents
  • Cooling coils
  • Drain pan / condensation line

Let’s take a look at these areas and common causes of mold growth in these areas in more detail shall we?

Mold and Air Ducts

HVAC air duct

When your air conditioning and heating system is running, dirt and mold spores from your home’s air are constantly circulating through the ducts. But your air ducts will really only start growing mold when there’s a regular source of moisture.

This makes moisture and condensation the largest cause of mold in HVAC ducts. Where does this moisture come from?

  • Roof leak—If your roof has a leak and your air ducts are in the attic, water may soak your ducts every time it rains.
  • Oversized AC unit—Large air conditioners won’t run as long and therefore won’t remove much humidity from your home’s air. The humidity can then condense on your metal vents and become a breeding ground for mold.
  • Plumbing leak—A hidden plumbing leak can let water into your ducts, creating a great environment for mold growth.
  • Inadequate bathroom venting—When you take a hot shower, that steam needs to go somewhere. Ideally, it should be vented outside by an exhaust fan. Otherwise it can collect on vents or registers in the bathroom.
  • During warmer seasons, cooler air is being piped through vents and sometimes moisture will condensate along the sides of the ductwork.

Intake Vents / Air Vents

If you are not diligent about cleaning those air vent covers, loads of dust, oil, and other debris can collect on them making it a hospitable location for mold to grow. It takes just a few minutes once a month with a microfiber duster to clean off those vents.

Mold On Cooling Coils

HVAC cooling coils

The evaporator coil (the part that cools your air) is the perfect environment for mold. It collects dirt and dust with gusto. In addition, moisture forms on the cold evaporator coil when warm air blows over it.

Cooling coils and drain pains have a close relationship. When moisture forms on the cooling coil it drips into the condensate drain pan, allowing the moisture to drain outside of your home or into your main sewer line. Or at least it should… more on that in a second. During the whole process, dirt and mold spores from your home’s air are also collecting on the coil and dripping into the drain pan. You see the issue here?

Mold In Drain Pans

Debris can stop or plug up the drains of the condensate drip pans. Not only can mold begin to grow in the drain pan but water can overflow from the pan and onto the floor or rooms below. Water overflows can cause extensive mold problems in the areas surrounding the HVAC system.

Other HVAC Mold Causes

Another common cause is improper sizing of AC systems. Your system may be over-sized or under-sized and this allows for inefficient operation of the entire system, which may lead to condensation issues.

An HVAC system that has a humidification unit attached to it is bad news. You do NOT want to purposefully introduce humidity into your home on a regular bases and especially NOT through your HVAC system. This is a mold disaster waiting to happen.

How To Prevent Mold In Your HVAC and Air Ducts

  • Ensure that your HVAC system in correctly sized to provide correct air flow and meet room-by-room calculated heating and cooling loads.
  • An HVAC should be installed so that the static air pressure drop across the handler is within manufacturer and design specifications to have the capacity to meet the calculated loads. It should also be installed with a return system sized to provide correct return air flow.
  • Have properly sealed supply ductwork that provides proper air flow.
  • Have sealed return ductwork that will provide proper air flow to the fan, and avoid air entering the HVAC system from high particulate zones (garage, attic, basement, and crawlspace).
  • Minimize duct air temperature gain or loss between the air handler and room registers, and between return registers and the air handler.
  • Drain pans should be sloped properly and checked monthly.
  • Use high quality HEPA AC filters and change them every season (more if you live in a dusty climate like the Arizona or Nevada desert).
  • Once a month, place a humidity meter over one of your main air vents to measure the humidity levels of the air blowing out. If the humidity reads 55-60%, you may develop a mold problem. You will want to determine the cause of the high humidity as quickly as possible.
  • Practice routine HVAC maintenance. The key to keeping your HVAC system in good shape is a bi-annual maintenance call with a qualified technician. If your system is regularly maintained, any signs that point to mold, such as leaks and pools of condensation, can be addressed before the problem becomes bigger than it needs to be.
  • Get yearly air duct cleaning. To prevent the growth of mold, it’s important to remove the food source in your air ducts. Not only will this cleaning remove any dust, dirt, and debris that could impact how well your ducts circulate air in your home, it will also remove any visible traces of mold and mildew that are present. A duct cleaning will also make your ducts an environment where mold cannot easily grow in the first place, reducing the risk of spores developing and spreading.
  • Treat coils, plenums, pans and duct work with a mold prevention spray like Superstratum Mold and Mildew Protectant. Spray down the first few feet of duct work, the coils, the condensate pan and pretty much anything else that may be impacted by heat and humidity. Superstratum will create a barrier that effectively stops any mold from settling down, therefore controlling any mold growth for months. Make sure to re-apply the next time you are doing a coil cleaning or other maintenance service. Superstratum has a 6 month, no mold guarantee when used for HVAC and air duct applications. Click here learn more about Superstratum.
  • Monitor humidity and run a dehumidifier to keep humidity low.
  • Use your AC on recirculate to limit how much outside air and particulates are being drawn in.
  • Periodically test your air ducts for mold using a mold test plate. You can read up on how these work here. But in short, swab the inside of your air ducts with a clean q-tip and transfer the dust collected onto one of these mold test plates. Follow the instructions on sending it in for analysis. If it shows mold growth, you will want to get to the root of the issue ASAP.
  • Dust your air vents regularly to keep dust and debris from building up on them.
HVAC unit outside

How To Remove Mold From An HVAC System

Mold remediation can be a challenging task when it affects HVAC components such as ductwork and air handlers. As such, I do NOT recommend a DIY approach and instead suggest you call an HVAC company and a professional duct cleaning company.

There are a few things you should be aware of when hiring a company to professionally remove the mold from your HVAC system and components. You want to make sure that they will do the following:

  1. Cover all the air vents. This is step one before any mold removal in the HVAC system or air ducts occurs.
  2. Turn off the HVAC system completely (including any fans running off the thermostat).
  3. Seal the intake and returns while working on the HVAC system and components.
  4. If there is standing water, a vacuum should be used to pull out all water from the system.
  5. When it comes to ductwork, porous components such as insulation and fiberglass ducts must normally be replaced when they are affected by mold. There is simply no way to remove mold from these items. Metallic ducts and other non-porous components are less susceptible to mold and can be reused with proper cleaning.
  6. While cleaning, it is best to individually isolate each section of ductwork removing the protective covering over each vent and intake as needed. Once isolated, cleaning can commence using a bladder system. This collects any spores that are knocked free and prevents it from re-entering the air supply or ending up in another area of the building. When the isolated ductwork has been cleaned, the area needs to be fogged with a disinfectant made for HVAC mold removal.

Drain pans and cooling coils are potential components of your HVAC system that you can take a DIY mold removal approach with. Before tackling either one, be sure to wear a respirator and gloves. It also wouldn’t hurt to wear some old clothes you don’t care about.

To remove mold from a drain pan, simply soak a cloth in a mold remover like Superstratum Mold and Mildew Stain Remover. Wipe the drain pan thoroughly then give it a second wipe down with a new cloth. You can pour the Superstratum down the drain line and use a soft bristled brush to scrub inside as far as you can reach. That’s about all you can do short of replacing the drain pan and line completely.

Cooling coils are a little bit tricky but still doable.

If possible, vacuum the coils gently with a HEPA vacuum or use a soft brush. This will help remove surface mold and dirt. Make sure not to damage the sensitive fins around the copper coils. 

Wet clean the coils with an alkaline coil cleaner and pressurized water. Many cleaners have acid in them and while it makes coils nice and shiny, it also strips metal off them. Also, “self rinsing” coil cleaners aren’t what they are hyped up to be. They deliver limited cleaning power and require condensation to actually work.  My recommend coil cleaner is this one.

Once completely cleaned of mold and dirt, make sure to apply a mold inhibitor like this one to the coils and surrounding areas, including the first few feet of duct work. This will provide some safety against regrowth until the next time you clean your coils. 

Will a UV Light Remove AC Mold?

HVAC UV light to prevent and remove mold

Using a UV light in your AC and duct work is not the best way to remove mold. It will leave you with continued mold growth – bottom line.

Ultra violet light breaks up the DNA of some types of mold which results in the sterilization of that mold.  The concept is great if it really were that easy. 

UV light only works on mold if the light is held 1-2 inches from the effected surface. Depending on the surface, it takes anywhere from 10 – 30 seconds in that area to effectively kill the mold.  As the distance the UV light is from the mold increases, the intensity of the light decreases and renders the process ineffective. 

This process when done correctly supposedly kills 99% of the mold.  Now let’s assume there is one million mold spores in an area. That means at the end of this process, there are still 10,000 living mold spores that are colonized and still reproducing in the “treated” area.  Not good.

A UV light to remove AC mold is a painfully slow and ineffective way to treat mold. UV light exposure is also harmful to people and all living organisms.  These lights emit ozone that is bad for the lungs, emit ultra violet radiation which is bad for the skin as well as damaging to the eyes and the immune system.

Don’t fall prey to those UV lights for mold removal. You will be worse off than you were without them.

In Conclusion…

Nothing can replace proper design, operation and maintenance of an HVAC system to prevent the growth and spread of mold. Be diligent about mold prevention and leak prevention in your home at all times. Prevention is your best defense against HVAC mold.

An HVAC unit that is mold free

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  1. Braden Bills says:

    I have an old HVAC system, and I think the vents might be dirty. I didn’t realize that not taking care of a vent could eventually lead to mold! It would be smart for me to get a professional to clean my vents out for me. They would be able to ensure that there isn’t any risk of mold in them.

  2. rachel frampton says:

    I’ve been stressing out about my duct line because it’s producing an awful smell. I never knew that humidity can cause molds to grow quickly and en masse, that’s probably the reason for the smell as well. Anyways, what’s important now is to look for a duct cleaning service that’ll be able to address this concern.

  3. Daphna Rubin says:

    We just broke our lease and moved from a condo with Stachy LAST WEEK. Today mold was found in my AC unit in my new rental on the coil and ceiing of the unit. AC guy cleaned the coils to remove the mold and wanted to apply microban to the mold. I refused. He installed UV light. How should the mold be removed? Is it safe to assume mold is in the ducts? Should I have the ducts swabbed? Air tests? Please help!

    1. I am so sorry to hear this! Before you do anything else, I would absolutely test inside the ductwork. An ERMI test would be your best choice for this. Depending on what that shows, I can advise you on to proceed. It may be a situation where you should unfortunately look for another rental.

  4. Joven Ruthford says:

    As the distance the UV light is from the mold increases, the intensity of the light decreases and renders the process ineffective.