Old and Smelly Sock
© Юрий Красильников – stock.adobe.com

When the seasons change, you might start looking forward to the scent of spring flowers or crisp fall air, but the odor of dirty gym socks moldering in a forgotten corner of a locker room probably isn’t on the list. For many homeowners, though, this unpleasant smell is exactly what shows up.

Known as dirty sock syndrome, it can be a frustrating situation, but it usually isn’t difficult to get rid of once you learn the underlying causes.

The Source of the Smell

Woman Smelling Something Bad
© khosrork – stock.adobe.com

“Dirty sock syndrome” is the term used for what happens when a heating and cooling system produces an unpleasant dank, musty smell reminiscent of dirty socks. In milder cases, the odor might be similar to the dusty smell produced when the system is first turned on after it hasn’t been used in a while. It’s mostly noticeable when the air conditioning kicks on or when the heat pump goes into the defrost cycle.

It might sound like something made up by a frustrated homeowner, but dirty sock syndrome is exactly what HVAC professionals call it. The cause, not surprisingly, is the same thing that makes long-forgotten dirty socks smell: a buildup of mold and bacterial slime.

The problem starts during air conditioning season, when the evaporator (indoor) coil is cool and damp, allowing microbes to thrive there. When you switch to heating in fall, the microbial slime dries out. As the weather grows colder, at some point, the HVAC system will run a defrost cycle in between periods of heating.

It’s this switching back and forth between heating and cooling that causes the mold and bacterial slime to emit a foul odor. It can also happen in spring when you switch between heating in the morning and late evening, and cooling in the afternoon.

You might notice the smell goes away when you run the system on only heating or only cooling for a long period, but that doesn’t mean the problem is solved. On the other hand, if your HVAC system produces unpleasant odors all the time, especially in heating mode, your problem is something other than dirty sock syndrome.

The problem can happen in any type of heating and cooling system that uses an evaporator coil, but it’s most common in heat pump systems. In gas furnace systems, the heat produced is usually intense enough to kill off any microorganisms.

Dirty sock syndrome tends to happen more often in newer HVAC systems. Some HVAC professionals have theorized that manufacturers have been using more porous aluminum for evaporator coils made in the last decade or so. This more porous surface provides more microscopic nooks and crannies where mold and bacteria can grow. Dust in the system further exacerbates odor problems by providing bacteria with food.

The good news is there’s nothing wrong with your HVAC equipment. It most cases it just needs a good cleaning and better maintenance in the future.

Mildew Odors and Your Health

Stethoscope and Heart
© natara – stock.adobe.com

It’s logical to assume mold and bacteria powerful enough to stink up the whole house would be a threat to your health, but in reality, dirty sock syndrome is rarely dangerous. At lower levels, the odor-causing microbes won’t affect someone with a healthy immune system.

That said, if someone in your household has a weakened immune system or respiratory problems, such as asthma, they could be at increased risk of harm. What’s more, if the mold spreads, it can start growing throughout your HVAC system, putting more mold spores into the air and creating a greater health threat that could affect otherwise healthy people.

So even if you can tolerate the odor for a while, it’s important to treat dirty sock syndrome as soon as possible before the problem spreads.

Getting Rid of HVAC Odors

Magnifying Glass
© christianchan – stock.adobe.com

Before you treat your HVAC system for dirty sock syndrome, make sure that’s actually what you’re dealing with. Foul odors can come from many sources, so inspect your system for other possible causes.

Clean the drain pan – Empty the drain pan under the indoor evaporator coil and check for blockages. If you can pour in a small amount of water and it pools instead of draining away, the condensate drain line is clogged. In this case, you’ll need to use a wet-vac to remove the clog via the outdoor cleanout line.

Inspect plumbing drain lines – Lines connected to plumbing systems can start to smell when the p-traps have dried out or deteriorated.

Look for duct blockages – Rodent nests, dead rodents, and insect infestations are all possible causes of odors.  

Look for duct leaks – Inspect the accessible parts of your duct system for loose or unsealed connections between sections of metal ductwork, tears in flex duct, and ducts with open ends that empty into wall chases or other open spaces. These can draw odors from the attic, basement, and walls into your rooms.

Check the air filter – If your A/C air filter is wet, you have a moisture problem that could well be causing the odors you notice. A clogged filter can contribute to odors, among other problems, so change the filter according to the manufacturer’s directions.  In most homes, a standard fiberglass filter should be replaced every three months.

If you’ve taken all these steps and you’re still getting that locker room smell every time the A/C kicks on, your system most likely has dirty sock syndrome and your evaporator coil should be cleaned. To treat a mild case, it might be enough to clean the evaporator yourself using a foaming coil cleaner. If this doesn’t solve the problem, contact a professional to have the coil cleaned more thoroughly. For a severe case, you might need a new evaporator coil.

Keeping Your System Smelling Good

Woman Smelling Something Good
© Drobot Dean – stock.adobe.com

Once you get your coil clean and your house smelling fresh again, some preventive measures will reduce the risk of the odor coming back. These steps benefit your whole HVAC system, so they’re well worth the investment.

If you’ve been using standard 1-inch fiberglass filters, upgrade to a more efficient synthetic media or cotton-polyester filter with a Minimum Efficiency Reporting Value (MERV) of 5 to 11. These trap more airborne particles, starving microbes of food.

Keep an eye on how well your air conditioner’s condensate drain works. If it frequently clogs or never drains efficiently, it might be damaged and need replacing.

Make sure the joints between the duct sections and points where the ducts connect to air registers are sealed with mastic or foil tape that’s in good condition. If there’s no sealing at these points or it’s deteriorating, reseal. Sealing your ductwork helps keep excess moisture and dust out of your ducts. It also improves your ducts’ energy efficiency, lowering your heating and cooling bills.

Contact an HVAC professional certified by the National Air Duct Cleaners Association (NADCA) and schedule a duct inspection. A properly installed duct system picks up little if any debris, so most only need cleaning every five to 10 years. Even so, there’s always a chance a problem such as a rodent infestation or water leak could pop up and start to produce foul odors. An inspection will tell you if your ducts are contributing to your system’s odor problems and whether or not they should be cleaned. 

Consider having a UV light installed in your system to shine on the evaporator coil and kill any microorganisms that might end up there.

As unpleasant as it is to discover your air conditioner is filling your house with the odor of stinky gym socks, the problem is rarely serious. Most cases of dirty sock syndrome can be solved with a thorough cleaning. Getting this taken care of as soon as possible prevents microbes from spreading through your HVAC system where they could grow into a much larger problem. Once the odor is gone, take steps to prevent any future mold and bacteria growth in your HVAC system.

Editorial Contributors
Henry Parker

Henry Parker

Henry Parker is a home improvement enthusiast who loves to share his passion and expertise with others. He writes on a variety of topics, such as painting, flooring, windows, and lawn care, to help homeowners make informed decisions and achieve their desired results. Henry strives to write high quality guides and reviews that are easy to understand and practical to follow. Whether you are looking for the best electric riding lawn mower, the easiest way to remove paint from flooring, or the signs of a bad tile job, Henry has you covered with his insightful and honest articles. Henry lives in Florida with his wife and two kids, and enjoys spending his free time on DIY projects around the house. You can find some of his work on Today’s Homeowner, where he is a regular contributor.

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