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When you switch on your air conditioner, you expect to feel it, not smell it.

Imagine you’re sitting at home on a hot day, watching something on TV. The AC kick in, you breathe deep and get a faint whiff of something slightly horrible. You look over at your spouse, significant other, or dog. They, in turn, look back at you. They look back at you with that look that says, “What in God’s name did you eat?” But it’s not you, it’s not them, it’s your AC and it’s wafting through the entire house.

In a majority of cases, air conditioner smells are caused by mold and mildew. As air circulates throughout your home, many airborne contaminants have the opportunity to settle within the system. Even when your air conditioner isn’t being used during the cooler months, dust and dirt can collect. Contaminants within your system combined with cool, damp conditions are perfect for mildew and mold to form in your equipment, and the smell can be carried into your home as the system circulates cool air.

Smells coming from your cooling system could also be the result of a dirty air filter. Your system’s filter works to pull odor-causing contaminants out of the air supply as it circulates. If you haven’t been changing your air filter on a regular basis, these contaminants could be recirculating back into your home when the air conditioner is on.

In some cases, you may think a smell is originating from the air conditioner when it’s actually being picked up somewhere else and circulated through the HVAC system. Chemical cleaners, exhaust fumes, perfumes, candles – these odors could be created somewhere else inside or outside the home and be circulated throughout the house via the ducts.

The first step is to check your system and make sure it’s not the equipment that’s causing the smell.

Even with normal use, dirt, dust, and other debris are deposited on the evaporator coils of your cooling system. These particles combined with the damp, cool environment of the evaporator coil create the ideal breeding ground for mold and mildew. Getting to the coils in your system is difficult and delicate. As such, we do not recommend you open your unit to look inside. Simply smell your unit and if anything smells “off” call in a pro.

The drain pan and condensate drain line which carry moisture out of your evaporator coils can become clogged. When moisture is unable to flow out of the system, mold and mildew can also form in these components.

Debris can also accumulate in duct work and the air is circulated. If moisture isn’t being properly controlled, the environment in the ducts can become damp.

Holes, cracks, and gaps in your ductwork can allow outside odors in. Mold and mildew as the result of a roof leak or wet crawl space could be seeping into your duct system through these openings and be the source of moldy smells, especially if your evaporator coils and filter are clean. If you have ductwork running through your garage, odors such as exhaust fumes and chemicals could be entering through these openings.

You should have your system cleaned, checked and maintained by professional on a yearly basis. The system’s evaporator coils should be cleaned annually to reduce smells, and procedure will also improve the efficiency of your air conditioner. In addition, duct cleaning will remove the contaminants that could be recirculated through the house.

As we mentioned above, air filters are a main culprit in many cases. You should make sure to change these regularly.

If you’ve taken these steps and the smell persists, you might consider installing an air purifier and ultraviolet (UV) lights in the duct work. An air purifier can remove airborne bacteria and UV lights installed in the ducts can eliminate the growth of mold and mildew in the ducts and on the coils by killing mold spores, bacteria and viruses in those area. Before you take these steps, reach out to an HVAC professional and get their opinion. Don’t have a trusted pro? We’ve put a list together here.


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