Air Conditioning Mold Issues: Top Blunders that Cause Unhealthy Mold


It’s been hot. Really hot. And to keep comfortable, we’ve been turning to air conditioning to mitigate the misery. But what many people don’t realize is that by turning on the A/C, they may be spreading trouble – trouble in the form of mold spores – and that’s not so cool. Mold can cause serious health problems including respiratory issues, headaches, fatigue, rashes, itchiness, and other allergy symptoms.

Here’s how mold develops in air conditioning systems and what you can do about it:

Mold in Window Air-Conditioning Units

Some window units have reusable filters that can be cleaned with soap and water, while others are disposable and need to be replaced every 3-6 months. A dirty filter can serve as a food source for mold, accelerating its growth. Many units have a “check filter” light that lets you know it’s time to change the filter. Don’t ignore it!

Even the most expensive window air conditioning units can develop mold. Over time, dust collects inside the vents and other parts. Add a little humidity and mold will begin to grow, feeding on the debris and particles of dust. Many of the parts now used to manufacture these units are plastic, and mold loves to grow on this material. Condensation also loves to gather on plastic. You likely wouldn’t notice the air conditioner’s mold problem until you turn the unit on in the warmer months and that musty mildew odor appears.

window ac moldVery importantly, always be sure to tilt a window air conditioner back so condensate produced will drain outside, NOT inside where interior walls and floors will become saturated. There is a drain hole, usually at the bottom of the back of the unit where it hangs outside that allows this drainage to occur. But you need to go one step further. Interestingly enough, this hole usually has a plug in it for shipping purposes. Be certain to remove the plug when the unit is installed. Otherwise, the condensate will be trapped at the bottom and drain inside the wall and into your room.

HVAC Mold Issues

Ever see a water stain on a ceiling but you’re not sure what’s causing it?  It could be your HVAC air handler in the attic. Ductwork, A/C evaporator coils, and drip pans are the perfect environments for mold to grow.

condensate overflow

So, what causes this? When the condensate pan gets full, water needs to move freely through the drain to exit the premise. Unfortunately, drains often get clogged with debris from rodents, like nuts and sticks or just an accumulation of minerals and other gunk from the water over time. Some units have an alarm that sounds when the water gets too high in the pan, but you shouldn’t rely solely on that because they have been known to fail. Once the water overflows, it travels to the lowest point it can reach, usually dripping through a ceiling or wall. Once these areas get wet, mold can grow in as little as 24-48 hours.

hvac mold preventionAnother common problem is condensation around vents that causes mold growth. When you keep the HVAC unit at colder temperatures, condensation can build up around the ceiling and wall vents when cooler, air-conditioned air hits the warmer air inside a room. You’ll see the mold growing around the vents on the ceiling or wall surface. Ductwork can also harbor mold from condensation and dust accumulation. It’s important to have your ducts cleaned about every four years depending on usage, whether you have pets, and the type of climate you are in.

HVAC Filter Faux Pas

Changing your filters regularly is also key in maintaining your HVAC unit. Most filters are made from standard fiberglass. They are relatively inexpensive but are not fine enough to catch mold spores. Consider upgrading to a high-efficiency filter with a MERV rating of 13 or more. The MERV rating indicates the size of particles the filter is capable of trapping. A MERV 13 will trap almost all the typical airborne contaminants, including dust, pollen, mold, bacteria, smoke, smog, and even virus carriers. Be careful though, for some older units a higher MERV rated filter may impede too much air flow, so check with your HVAC professional if your air handler is more than ten years old.

how often should i change my hvac filterIf you see or smell mold coming from a vent or A/C unit, the best course of action is to turn off the system immediately, then call RTK for independent mold testing. It’s best to have the system tested by a professional mold inspector to prevent cross-contamination in the rest of your home.

Since RTK does not remediate and only tests for mold, there is no conflict of interest. Their comprehensive inspection will ensure you have unbiased results that will determine the best way to handle any mold issue. Call RTK at 800.392.6468.



Past exposure to lead may be to blame for over 400,000 deaths in the United States every year (The Lancet Public Health.)

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