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Why Does My Air Conditioner Smell Musty?

An air register that's blowing air from a musty air conditioner

When we turn on our air conditioners, we expect our homes to become cool and comfortable. In some unfortunate situations, however, homeowners turn on the AC, only to detect an unpleasant odor. If your air conditioner smells musty, you’re probably trying to get to the bottom of the problem and figure out what you can do to fix it.

The most common culprit of an air conditioner that smells less than fresh is the presence of mold somewhere in the system. The other possibility is that your AC unit’s coils have started to corrode. Either way, your air conditioner has likely experienced a buildup of moisture, and the warning sign that that this may be the case is a bad smell.

Not all musty smells coming from your vents start in your air conditioner, however. Identifying the cause of this problem is the first step to resolving it. There are a couple of signs that your AC unit is the root of the issue.

First, think back to last winter. Did you notice the musty smell in the cooler weather? If your bad smell is tied to summertime, then it’s more likely to be connected to your AC unit. The reason for this should be obvious: You’re more likely to have the AC running frequently during the summer, so bad smells linked to mold in the unit will be far more noticeable when it’s hot out. If the smell happens year-round, however, the problem is more likely to be in your vents instead.

If you’ve recently upgraded your AC system, you may actually be at higher risk for bad smells. This is because newer AC units use aluminum coils, which can corrode far easier and can emit undesirable odors. So, if the musty smell started after you got a new air conditioner, that is the likely cause.

Finally, if you’ve had any work done in your attic recently, your air conditioner might not be the cause of the problem. This is because careless contractors can accidentally put holes in your ductwork when working in your attic. Ductwork holes in the attic can lead to a variety of problems—a bad smell being just one of them.

If it turns out your AC unit is the cause of your bad smells, then you’ve run into a problem known in the industry as dirty sock syndrome. When there is a buildup of moisture in your air conditioner, it can lead to mold growth in your vents as well as to a smell that is reminiscent of dirty gym socks. There are a couple of reasons why you may have dirty sock syndrome, including:

  • Poor airflow: In order to work properly, your air conditioning unit needs a certain amount of air to flow through it. If your unit isn’t getting sufficient airflow, perhaps because an air register is closed, it can lead to excess moisture in your unit and, therefore, a musty smell. Another indication that your unit isn’t getting enough airflow is if your AC is blowing warm air.
  • Dirty air filters: These important components in your cooling system are designed to catch debris like pollen, dust and pet hair. When a filter is clogged, it provides an ideal environment for all sorts of unpleasant bacteria and mold. If left ignored, you’ll eventually notice a dirty sock smell because of this build-up.
  • Holes in the ductwork: If your home’s ductwork has holes, debris and smells from the attic can get sucked into your AC unit. As we mentioned previously, holes in ductwork can occur when you have a contractor working who may inadvertently leave these passageways damaged. Unfortunately, even if you haven’t had any work done on your home recently, there could be rodents living in your attic that are chewing holes in your ductwork. In order for this problem to be resolved, you’ll first have to have the rodents removed from your home. Then, you can get started on fixing your ductwork.

Fortunately, it is relatively easy to avoid many of these problems. By signing up for ongoing air conditioner maintenance, you can have peace of mind that a licensed specialist will inspect your unit for any signs of problems, including those that lead to a musty smell. Until help arrives, you may want to learn how to get rid of that smell.

A white kitchen with a granite countertop

How to Remove Odor from Air Ducts

In order to get your home smelling normal again, you may have to take all of the following actions. If you still can’t get to the bottom of the issue after doing so, your best bet is typically to contact a licensed professional who can troubleshoot your AC issue and recommend any needed repairs.

Replace or Clean the Air Filter

The first thing you should check for almost any AC problem is the air filter, since changing or cleaning your air filter can actually resolve many AC problems. Plus, if a dirty air filter is the cause of your problems, it should be obvious: The clogged filter itself will smell intensely musty and may have visible mold on it. If this is the case, go ahead and replace it. You should notice an improvement in the smell coming through your vents in an hour or two.

Most households should change their filters about every month to every six months, depending on the type and quality of filter. However, if you have pets, your filters can clog quickly. Furthermore, if you have young children or if someone in your family has respiratory problems, you may want to replace your filter on a more regular basis.

Clean Evaporator Coils 

Dirty evaporator coils can quickly lead to a bad smell and a variety of other issues, such as a frozen air conditioning unit. While professionals can handle this AC service quickly, it’s also possible to do it yourself. However, this is typically not a project that is recommended for homeowners who aren’t well-versed on the way air conditioners work. If you do decide to take this project on yourself, start by grabbing a pair of safety goggles and gloves.

To get to your evaporator coils, you will need to turn off the AC and get into the unit. The back of your AC unit should have an access panel that allows you to reach the coils. Remove that panel and set it aside. If you see a lot of dust and debris inside the air conditioner, use a small, handheld vacuum to remove it. You should also carefully remove the fins that protect the coils and carefully lean them up against a wall.

Next, get a spray bottle, bleach, water and a sponge. Fill the spray bottle with a 10% bleach solution—this is strong enough to kill mold and bacteria, but weak enough that it won’t harm your unit. Spray down the coils with the bleach solution and let it sit for a few minutes. You’ll be able to access the condensation collection pan, too, so while you are at it, pull that out and spray it down as well. Then wipe everything down with the sponge. Put the unit back together and let it run for a while to see if your smell problem is gone.

Check the Condensate Drain Line

The condensate drain line is designed to drain moisture from your AC unit. If it gets clogged, then moisture will start to build up over time, which can lead to mold and corrosion. If your condensate drain line is clogged, you’ll probably see water accumulating around your AC unit. If that’s the case, you need to unclog the drain line to prevent damage to the unit and stop further mold buildup.

Keep reading for step-by-step instructions on how to remove these types of blockages.

Water leaking from an air conditioning unit with a clogged condensate drain line

Fixing a Clogged Condensate Drain Line

A bad smell, a water leak from your AC unit and a full drain pan are all signs of a clogged condensate drain line. If you have one or more of these problems, you’ll want to unclog your drain line. You can either contact a professional heating and cooling specialist or take on the project yourself.

If you decide to attempt the do-it-yourself route, you will need:

  • Rubber gloves
  • Vinegar
  • Water
  • A bucket
  • A thin, stiff wire brush
  • A wet/dry vac
  • Duct tape

First, turn off your AC unit and locate your drain line. Your drain line should be a rubber or plastic tube on the outside of the unit. Place your bucket under the drain line and then use the wire brush to clean out any blockages at the end of the drain line. Sometimes this will solve your issue, but typically you’ll need to do a little more, especially if your air conditioner is smelling musty.

If you decide to keep working on the issue yourself, you’ll want to find the top of the drain line. This part should be attached to a T-shaped vent that enters the AC unit indoors. Remove the cap from the top of the T, put on the rubber gloves and pour a quarter of a cup of distilled vinegar down the drain. This liquid will drain into the other end of the line, into the bucket you already placed. After half an hour, pour water down the line to ensure everything is flowing out like it’s supposed to. Otherwise, it’s time to use the wet/dry vacuum to try to suck the clog out. Take the end of the wet/dry vacuum hose and attach it to the end of the drain line. Use the duct tape to seal the two together. Run the vacuum for a minute or two and see if this pulls the blockage out.

There’s a reason most people turn to the pros when dealing with these kinds of issues. If you go through all of this trouble and none of these tips unclog your drain line, you’re going to have to reach out to a specialist anyways to work out a more permanent solution, which can feel like a waste of time if you have already invested your precious free time on a DIY solution. A heating and cooling professional will be able to identify the cause of the stubborn blockage. Then, you can start a program of routine maintenance to prevent this problem from occurring again.

ABC Can Keep Your Home Cool and Comfortable

Eliminating bad odors coming from your air conditioning unit can be a long, complicated process if you don’t have the proper tools or expertise. When you contact ABC Home & Commercial Services about dirty sock syndrome, you can feel confident that our licensed specialists will be able to quickly determine the cause of the issue and then get to work on resolving the problem. In addition to AC repairs and maintenance, you can also rely on us for honest advice on when it may be time to replace your unit if your unit has reached the end of its lifespan or if the cost of continued repairs starts to approach the cost of a new unit.

Tom Riggs

Tom Riggs is the Division Manager for Mechanical Services, overseeing sales and operations for HVAC, Plumbing, Electrical, Appliance Repair and Water Quality for all ABC Austin branches. He joined ABC in 2014. Before ABC, he was an HVAC Service Technician, HVAC Comfort Advisor/Sales and Operations Manager. Tom attended Universal Technical Institute. He's an avid outdoorsman and enjoys country living with his wife and two sons.

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