It happens every year, twice a year. The seasons change, and you flip your cool-air setting to heat (or vice versa), and your HVAC system blows an awful smell into your home.
Some people call this phenomenon “dirty sock syndrome,” and it can be frustrating. Let’s explore what it is, why it happens, and how to avoid it.
What Is Dirty Sock Syndrome, and Why Does It Happen?
HVAC professionals use “dirty sock syndrome” to describe any dank, musty smell emanating from an air conditioner or heating system. It’s mostly noticeable when an air conditioner cranks up or when a heat pump enters its defrost cycle. It may resemble the dusty or burned air scent from a disused air conditioner.
Dirty sock syndrome starts from the same thing that makes dirty socks stink: mold and bacteria buildup. Microbes can thrive during air conditioning season when the evaporator coil is cool and damp. When you switch to heating in the fall, bacterial slime dries out.
Your system will sometimes run a defrost cycle between heating periods. This back-and-forth between heating and cooling causes a foul odor.
Dirty sock syndrome happens more often in newer HVAC systems. Some professionals have theories about why that is. One of them is that manufacturers’ greater use of porous aluminum leaves nooks and crannies for microbes to sit in. Dust in the system exacerbates odor problems by feeding the bacteria.
You might notice the smell goes away when you run your system on heat only or cool only for an extended period, but that doesn’t mean the problem is solved. If your HVAC system constantly produces unpleasant odors, especially in heating mode, your problem may be something other than dirty sock syndrome.
Mildew Odors and Your Health
Mold and bacteria powerful enough to stink up your home threaten your health. Although odor-causing microbes don’t pose a significant health risk for someone with a healthy immune system, they’re an elevated risk for anyone who is immunocompromised or has respiratory problems like asthma. Mold can grow throughout your ductwork, releasing more spores into the air and creating a greater health risk.
Even if you tolerate the dirty sock odor for a while, you should treat it as soon as possible before the problem spreads.
How To Get Rid of HVAC Odors
Before you treat your HVAC system for dirty sock syndrome, make sure that’s what you’re dealing with. Bad smells from air conditioners can come from many sources, so inspect your system for other possible causes by following these steps:
- Clean the drain pan. Empty the pan under the indoor evaporator coil and check for blockages. If you pour in a small amount of water and it pools instead of draining away, that signifies a clogged condensate drain line. 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 dry out or deteriorate.
- Look for duct blockages. Examine ducts for rodent nests, dead animals, or insect infestations that could be causing odors.
- Check for duct leaks. Inspect the accessible parts of your duct system for loose or unsealed connections, tears in flex ducts, and ducts with open ends that empty into wall chases or other open spots. These can draw odors from your attic, basement, or walls and push them into other rooms.
- Examine the air filter. A wet or clogged filter can contribute to odor issues. Replace standard fiberglass filters every three months or as recommended by the manufacturer.
If these steps don’t resolve the issue, you likely have dirty sock syndrome, and it’s time to clean the evaporator coil. To treat a mild case, use a foaming coil cleaner according to the package directions. If that doesn’t solve the problem, have a professional do a more thorough cleaning. In extreme cases, you may need to replace the evaporator coil.
How To Keep Your HVAC System Smelling Fresh
Once the system is clean and odor-free, preventive measures can help keep it that way. These steps are well worth the investment since they benefit your whole HVAC system:
- Upgrade your air filters: Switch from standard 1-inch fiberglass filters to more efficient synthetic media or cotton-polyester filters with a Minimum Efficiency Reporting Value (MERV) of 5 to 11. These trap more airborne particles, reducing food sources for microbes.
- Maintain the condensate drain: Regularly check the air conditioner’s condensate drain for clogs or inefficient drainage. Replace damaged drains promptly.
- Seal ductwork: Seal joints between duct sections and connections to air registers with mastic or foil tape. Tight seals prevent excess moisture and dust from entering the ducts and improve energy efficiency.
- Schedule a duct inspection: Have your ductwork professionally inspected by a National Air Duct Cleaners Association (NADCA) certified company. Properly installed ducts pick up little debris, so most need cleaning every five to 10 years. Even so, there’s always the chance a rodent infestation or water leak could pop up and start causing bad smells.
- Add a UV light: Your HVAC company can install a UV light that shines on the evaporator coil and helps kill microorganisms.
