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Odors In The A/C: How To Troubleshoot Smells

If you know someone who tells you that his or her vehicle’s air conditioner blows out air that smells like the inside of an old gym sneaker when the A/C is turned on, they are not alone.


If you know someone who tells you that his or her vehicle’s air conditioner blows out air that smells like the inside of an old gym sneaker when the A/C is turned on, they are not alone. Literally.


The cause is microbes that are getting a free ride while growing on the A/C evaporator.
Molds like damp, dark places, and various bacteria can also thrive under these conditions. But besides smelling bad, these organisms can be unhealthy to breathe.

Air conditioner odor complaints seem to keep coming back no matter what treatment vehicle owners try, right? The reason is that most odor treatments are short-term, while the source of A/C odor is a combination of factors, including airborne microorganisms, that naturally repeat over time. Many products that are offered to control odor, such as disinfectants, may have a relatively effective initial application. Unfortunately, even the strongest disinfectant leaves no long-term, residual protection against the inevitable repeat odor infestation.


Causes of A/C Odor

Air conditioning odors typically result from uncontrolled growth of bacteria and other microorganisms in the evaporator. Research by independent laboratories has found fungi such as Aspergillus, Cladesporium, Penicillium and others growing on cooling coils and other A/C system areas.

How do these microorganisms get there? They are naturally present in outdoor air and are drawn into the evaporator during air conditioner operation.

Moisture also contributes to A/C odor formation. Most of the moisture that condenses on cold surfaces inside the system drains harmlessly over time.

However, some moisture continues to cling to coil surfaces after the A/C shuts down. Microbial contaminants in the air combine with this lingering moisture in the evaporator. The warm, dark and moist conditions in the evaporator provide an ideal breeding ground for bacteria, mold and mildew to grow and generate musty odors.


Downsized Evaporators Trap More Moisture

As vehicles were downsized while also increasing their glass content, A/C system manufacturers reduced evaporator size while simultaneously increasing fin count. These densely packed evaporators trap and hold more moisture.

To see this for yourself, dip your hand into a glass of water. As you remove your hand from the water, quickly spread your fingers apart. No water will bridge the gap between your fingers. Dip and remove your fingers again, this time spreading them slowly to only a fraction of an inch apart. Water will bridge the smaller gaps between your fingers and remain there far longer than if you open your fingers wide.


Discovering an A/C Evaporator Odor:

A musty odor may be emitted from the air conditioning system of some vehicles that are usually operated in areas with high temperature and humidity. It’s most noticeable when the air conditioner is first turned on after the vehicle has been parked for several hours.

The odor could result from one or more of the following conditions:

1. Blockage of the evaporator housing drain pipe, resulting in a buildup of condensation.

2. Microbial growth in the evaporator, arising from dampness in the evaporator housing where the cooling airflow is dehumidified.

To address excessive air conditioning evaporator odor, check the evaporator housing drain pipe for blockage. If no problems are found, the evaporator and housing should be cleaned and disinfected.


Did You Know…

If you must replace an evaporator, you may want to look into finding one that has a special chemical coating that inhibits the growth of mold and bacteria.

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