Make Sure Your AC… is Mold Free.

Jeff Bookout
April 19, 2022

Reading Time: 5 minutes

After a long winter, we are likely all looking forward to warmer temperatures and spending time outside in spring, but with warmer temps also comes the need to kick on your AC unit. Now is a great time to do some spring cleaning to get your AC ready to be mold-free for the season. Our air conditioners make our homes so pleasant to be inside on a hot day, but because they also circulate air, you’ll want to ensure what you are circulating is mold free. So let’s look at three common types of Air Conditioning: Central Air, Evaporative Cooler, and Split Units.

Central Air

Central air conditioners or “Central Air” for short produce great cool air, but it might surprise you to realize that they also produce condensation. Condensation, even a small out of it, can do just what any other water source can do – grow mold.

There are two main parts of your air conditioner that I will discuss, the Condenser on the outside of your home and the Evaporative Coil which is your inside unit. In short, an AC system works by blowing air over the A-Coil which has become cooled by the condenser (see “F” in the picture below).

The parts that make up an Central Air Conditioning Unit. Illustration Courtesy of


Check the Evaporative A-Coil

The A-Coil is made up of small metal fins and the unit is shaped like an capitol “A”.  Since the coils are cold, condensation will collect on those coils and any excess will collect in a drip pan. However, almost invariably there can be mold all over these coils. Here is a picture of a home I recently inspected. You can see the drip pan is full and there is mold all over the fins.

Gross right? This is an example of mold under the A-Coil. There is a condensation drain pan (below) and above it you see mold growth all over the metal fans.


The problem with many systems is the drain pans are poorly designed. Instead of putting the drain hole in the bottom of the pan. They’ll stick the hole in the side of the pan. So water can’t get completely out and you’ll have standing water in the unit and where there is standing water there is mold.

A-Coils can be tricky to get to, so unless you are very comfortable with your unit, contact an HVAC (Heating, Ventaliation, and Air Conditioning) technician to come check it out. If you have a tech out and they do find a problem, besides cleaning the coils, see about getting a drain put in the bottom of the drip pan instead of the side.

Check the Plenum Box

If there is a problem in that A-Coil area, there’s a box directly above it or beside it called the Plenum Box. It is wrapped in insulation and that insulation can house a lot of mold.

Locate your inside unit and look at the metal casing above or beside the A-Coil. If you see black spots on that casing, that is mold. This is a problem and I see it all the time. I will open the attic, crawlspace, or closet and there is mold all over the metal. If you see it on the outside of the unit, that is a good indicator that there is mold inside and on the insulation. If you find mold on the outside, contact your HVAC provider and get your unit cleaned thoroughly.


Black Mold Spots on the outside of a Plenum Unit


Split Units

Split units are wall-mounted and more sophisticated than what you have in a typical “window” unit. They are long and skinny and typically mounted on a wall near the ceiling.

A wall mounted split unit air conditioner.


Inside the unit, there is a “hamster cage wheel” that forces air out. Condensation will collect on that wheel, and also on the outlets, especially if the mounting was not done properly and there is not enough pitch to allow it to drain out. Because of the condensation, I’ve seen mold growth all over these units.

Mold covered vents on a split unit


So very difficult to see and very, very difficult to find if you don’t know what you’re looking for. When I do an inspection, I pull the louvers down so I can see that hamster cage. And I’m looking for green on that hamster cage. I would say that 50% of the time I look at one of those, I see mold growth. If you see mold or something on the vents, you can use HavenWipes or BioBalance Peroxide to treat the vents. 

Evaporative Coolers

The most common mold problem with air conditioning in the homes we inspect comes from evaporative, or “swamp”, coolers. They work by pumping water from a pan onto pads and forcing air through the wet pads. While these are great for dryer climates, they give plenty of opportunities for mold. Mold can grow in the standing water at the bottom of the pan. Mold can grow on the fiber pads. To top it off, homeowners often don’t bother to check for mold since the unit is on the roof. These units can blow nasty air in the home and you wouldn’t even know it.

“How a Swamp Cooler Works” courtesy of Landmark Home Warranty.


If you have a serious mold problem you should consider replacing your swamp cooler with a better alternative, however, you can keep your swamp cooler mold free if you clean the pads and pan religiously. Clean monthly, if not weekly. Drain the water from the pan and put in fresh water, and change the pads. (I recommend getting the inexpensive pads as you’ll be changing them often). If you do that, you’re probably not going to have salt buildup or significant mold growth. But if you forget (an easy thing to do), that water will sit in the pan and in just a couple of weeks your evaporative cooler can turn into a mess.

Another tip if you are using a swamp cooler is to not ignore your nose. If you come in your house and smell a musty or fishy odor, it’s time to give your swamp cooler a good cleaning.

Here’s to Mold-Free Cooling

I hope this guide was helpful. As always, if you have questions about clearing up mold in your home reach out to us at BioBalance. I hope you enjoy the warm weather and are able to stay cool inside your mold-free home.