How To Prevent Mold in Air Conditioners? (4 Step Guide)

Do you want to know the best ways to prevent mold growth in your central air conditioner or HVAC system? 

You are on the right website.

I wrote this article from my perspective as a licensed home inspector who has seen many homes with mold problems. In this guide, I will go over...

  • 4 steps to prevent mold in your HVAC system
  • why an UV light kills mold in air conditioners
  • how to "fog" your a/c system with a breathable anti-microbial spray
  • why periodically cleaning your air ducts can help prevent mold
  • the basic signs that mold is in your system
how to prevent mold in air conditioner

Mold can be a scary thing to have in your air conditioning system, but it is a "solvable" problem that can be cleaned and prevented. In fact, a recent study showed that a simple UV light killed 98% of bacteria in a hospital.

Let's get started with this guide!

What is the Cause of Black Mold in the HVAC?

Mold spores --- a variety of species --- are naturally present in the air. This is why bread will quickly have mold growing on it after it spoils. There are only three things that mold needs to take root and grow...

1. Food source

Mold needs a food or energy source in order to grow from the spore level. In the HVAC system, it is in the form of dust. Dust is mainly composed of dead human skin cells, but will also be clothing and carpet lint, and other debris. If you have never had your air ducts and furnace cleaned, the dust will just get thicker over time --- a ticking time bomb if other conditions become active.

2. Moisture

Besides an energy source in the form of dust, mold also needs water or moisture in order to grow and thrive. There are two primary areas in the HVAC system where moisture occurs. The first is at the air handler (or furnace) --- this is where your evaporator coil, blower fan, and possibly furnace is located. During the air conditioning process, a large of amount of moisture is extracted from the air, and this water is drained away to a drain.

If you see 3/4" PVC piping coming out of your air conditioner inside your home, this is the condensate (water) drain line. And if your air conditioner is draining very well, has other issues, or you just have alot of humidity in your system... molds will have an abundant supply of moisture.

3. Darkness

Sunlight is a natural disinfectant. If you have ever watched a survival show on the Discovery Channel, you may of learned how you can put water in a clear plastic bottle or plastic bag, put it out in direct sunlight for a few hours, and it will naturally kill microbes.

Unfortunately, inside of your air conditioning unit, there is zero direct sunlight --- it is almost the perfect habitat for microbes.

Read Also: Does UV Light Kill Mold? (How Effective?)

mold in a/c

Step 1. Clean Your Air Ducts

Air duct cleaning is a great way to remove dust from your HVAC system in order to minimize the chance of mold growing inside. And if you already have mold, duct cleaning will remove the mold from your central air conditioner.

Air duct cleaning is the process of connecting a heavy duty vacuum to the air ducts. The large vacuum is sometimes outside as part of a truck, or it may be a portable vacuum inside. 

A large diameter hose, such as 12", is connected to a return duct --- and then the ducts are cleaned using compressed air tools or brushes. When the vacuum is turned on, it creates a significant negative pressure in the duct system, so that when dust is loosened within the ducts --- it all gets sucked to the vacuum.

If you have your air ducts cleaned, it is important that they also clean your indoor air handler or furnace. This is the central area where all the air in the home goes through, this is also usually where your filter is located (though sometimes it is at the return duct vents). It is important to clean the blower fan, the evaporator coil, the furnace housing --- the whole unit. 

Almost always, if I find mold in a house, it is usually at the indoor air handler.

Step 2. Install An HVAC Ultraviolet Light

An ultraviolet light is a great way to prevent mold from taking root in your air conditioning unit. UV lights use a special light frequency that has been proven to kill mold and other microbes. The UV light is installed on the indoor air handler, and the light shines inside the unit 24 hours a day.

Most UV lights will have a special viewing hole so that you can verify the UV light is actually on. It is important not to turn on the UV light before it is installed because the light can damage your eyes and even your skin.

If you have black mold in your central air conditioner, you really should clean the existing mold before installing a UV light. But after it's cleaned, a UV light is a great way to ensure that black mold doesn't grow back.

The location where you install UV light is also important. You want to make sure the UV light has the maximum surface area where it is installed. You want the ultraviolet light to shine upon the evaporator coil, the blower fan, the housing, and other parts of the air conditioner or furnace.

Read Also: How To Install a UV Light in Your HVAC (Is It Easy?)

Step 3. Fog The Air Ducts With Plant-Based Sanitizer

Another technique that helps to prevent mold buildup within an a/c system is to periodically fog the duct system with a breathable anti-microbial spray. You want to make sure that the disinfectant is breathable, preferably plant-based, and is safe to use. Black mold in an air conditioner is known to be related to numerous diseases.

There are cold fogging machines on the market that you can purchase for homeowners. You simply pour the disinfectant into the fogging machine, plug it in, and point the spray nozzle at one of your return ducts for a few minutes.

You will want to first make sure the a/c is on so that the mist will go throughout the entire duct system.

If you actually own the fogging machine, you can easily fog your air ducts once every few months, or even once a year.

If you have any doubts about using a fogging machine, it would be best to contact a qualified air duct cleaning company.

Step 4. Stop A/C Water Leaks

If your air conditioner is not draining condensate or water properly away from the air handler, then the moisture may build up and increase the likelihood for mold to grow.

Air conditioners will have an evaporator coil with a drip pan underneath it. If the drip pan isn't sloped correctly, or if it isn't draining properly, then problems may develop. It is important to make sure the 3/4" PVC condensate drain pipes aren't clogged and draining away from the unit.

One thing you can do is turn on the air conditioner for 10 minutes, and then take off the cover where the evaporator coil is located. If the water isn't draining properly, you will easily be able to see it.

During my home inspections, I frequently see condensate drain problems and visible evidence of rusting of the A-coil and even the drip pan --- meaning the water isn't properly draining. Sometimes this excess moisture can develop into black mold.

mold in a/c

What's The Bottom Line On Preventing Mold In Your A/C?

If you ever have mold in your a/c system, or seen an air conditioner with mold, then you know that doing everything possible to prevent mold growth is a priority. We discussed having your air ducts periodically cleaned, installing a UV light, and making sure that the condensate (water) is properly draining away from the indoor unit.

Besides this practical ways to prevent mold growth, probably the most important thing you can do is open up the cover to your indoor air handler or furnace once in a while just to make sure there isn't any mold growth or other problems.

Also, once in a while, take a flashlight and inspect your air vent covers in each room of the home. If you ever see a black mold-like substance, there is a good chance that there is mold in your indoor air conditioning unit. During my home inspections, those are really the only two locations that I see mold growth within an air conditioning system.

Read Also: 7 Pictures Of Mold In Air Ducts

Meet Your Home Inspector Secrets Author

2 thoughts on “How To Prevent Mold in Air Conditioners? (4 Step Guide)”

  1. My evaporator coil has sprung a leak for the third time. It was brand new in May, 2015. One A/C contractor said the UV light is to blame. What is your opinion please?

    • Hi Susan,

      If you are talking about a refrigerant leak, then I don’t think the UV light is to blame. UV light sometimes degrades plastic materials such as the AC pan, electrical wire insulation, or even filters — but the evaporator coil is made out of copper and aluminum. UV lights have been extensively tested by researchers over the years in air conditioners (

      However, if you are talking about a condensate (water) leak, then it is a possibility, and you can either try a different UV light (lower wattage) or try a different UV light location.




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