A musty smell coming from your air conditioner is usually caused by moisture buildup, clogged drain lines or mildew growth. While some fixes like replacing your air filter can be done on your own, ongoing odors are best handled by a professional HVAC technician.
Have you noticed an unpleasant, musty odor in your home when you turn on the air conditioning? It may smell like dirty socks, a damp odor or another foul smell.
You’re not imagining things. There are a number of reasons why your AC can create a musty smell in your home. Even if the smell isn’t too strong, it’s important not to ignore it. The odor can eventually lead to bigger problems, such as poor air quality, mildew and low efficiency.
This comprehensive guide covers everything you need to know about musty air conditioning odors, including causes and fixes. Additionally, this guide includes expert insights from Gary Cremeans, ABC Home & Commercial Service’s HVAC Operations Manager.
What Causes a Musty Smell in Your Air Conditioner?
According to Gary Cremeans, there are several possible causes of the musty smell in your air conditioner. Cremeans points to the evaporator coil as a primary culprit.
“The coil is cold, dark, damp and porous. The perfect incubator for growth. The coil can absorb VOCs (volatile organic compounds) and odors in the air, then start to smell. Your soaps and smell-good stuff all collect and interact on the coil.”
Moisture Buildup Inside the Evaporator Coil
The role of the evaporator coil in an air conditioning unit is to remove heat and humidity from your home as it cools the air. Since Texas experiences high humidity, evaporator coils can produce a lot of condensation.
When the stagnant water doesn’t drain, the evaporator coil remains damp for too long and can produce a foul odor. Eventually, the musty smell can turn into mildew.
Signs of this problem include damp spots near the air handler, water pooling around the drain pan or the AC shutting off on its own. Contact a professional to inspect your unit if you suspect you’re dealing with moisture buildup.
Clogged or Dirty Condensate Drain Line
Another part of your AC unit that’s impacted by moisture buildup is the condensate drain line. Dust, algae and debris can clog the drain line and trap moisture. Eventually, it can create a musty smell and overflow water near the indoor unit.
Mildew in the Air Handler or Ductwork
Texas’ warm and humid climate creates ideal conditions for mildew and bacteria to grow quickly in your air conditioning unit.
Mildew in the air handler or ductwork can create a strong, foul odor when your system starts. The smell will likely fade once the unit has been running for a while, but you should still contact an HVAC specialist to inspect your unit.
Dirty or Wet Air Filters
Air filters are designed to trap debris, dust, pollen and other particles to maintain your home’s air quality. If your filters are overdue for a change, they may also be trapping moisture. This issue is especially common during the warm seasons in Texas.
A filter that’s damp with clogged moisture can create a musty odor, restrict airflow and work less efficiently. In general, air filters should be replaced every three months. If you have pets or excessive dust, you may need to replace it more often.
Oversized AC Units Short-Cycling
If your air conditioning unit is too large for your home, it may shut off on its own before it removes all the humidity from the air. This causes a problem called short-cycling. Keep in mind that short cycling is not the same as your AC unit tripping the breaker.
Excess humidity in the air can create moisture buildup in your AC unit, which can lead to odors and mildew growth.
Standing Water Around the Outdoor Unit or Attic Unit
Some of the issues mentioned above can cause standing water to collect around your AC unit, especially in units located outdoors or in an attic. Standing water creates the perfect conditions for mildew and bacterial growth, which create odors.
No matter the cause of the musty odor, it’s important to work with an HVAC specialist to get to the bottom of the issue before it leads to bigger problems.
Risks of Ignoring a Musty AC Smell
The risks of ignoring a musty AC smell include:
- Irritating allergies
- Mildew spreading into your air ducts and living spaces
- AC system efficiency loss, which often leads to higher energy bills
- Structure concerns due to condensation leaks in your attic and walls
How to Fix a Musty-Smelling Air Conditioner (Step-by-Step Guide)
There are several things that homeowners can try on their own to fix a musty-smelling air conditioner:
- Replace the air filter
- Inspect and clear the condensate drain line
- Clean around the indoor unit
- Check the thermostat settings.
However, sometimes DIY solutions aren’t enough, and you need to call in the professionals.
Disclaimer: Homeowners should avoid opening sealed HVAC components.
Replace the Air Filter
The easiest solution is to replace your air filters, which can get clogged with debris and moisture. In general, replace your air filters every three months. However, homes with pets, allergies or high AC usage may need to replace their filters more often.
For Texas homes, air filters with a MERV rating between 8 and 11 are ideal. Replacing your filters on time is the quickest way to improve indoor air quality.
Inspect and Clear the Condensate Drain Line
A simple way to check for a clogged condensate drain line is to watch for water dripping from the drain during a cooling cycle. If no water appears, it could be due to trapped moisture and debris.
Follow these steps to clear the condensate line:
- Turn off the AC unit
- Locate the condensate drain access point, which is usually a small PVC pipe near the indoor unit
- Pour one cup of white vinegar into the line
- Let the vinegar sit for 30 minutes before turning the unit back on
The vinegar will help break up the buildup in the line so that moisture can flow freely again.
Clean Around the Indoor Unit
Some light maintenance around your indoor unit can help with musty odors. Vacuuming up dust, removing any items that block airflow and ensuring adequate ventilation can go a long way.
Check the Thermostat Settings
Improper thermostat settings may be causing the musty odor in your home. For Texas homes, 45 to 55 percent humidity is the ideal setting. Avoid setting the thermostat too low, as that can cause foul odors and coil freezing.
When DIY Isn’t Enough
If you notice these issues, they could indicate a deeper problem:
- The musty smell persists after an air filter change
- There’s water in the drain pan or your air conditioner is blowing out water
- You can see mildew
- Your AC unit is struggling to maintain its temperature
Professional Solutions HVAC Technicians Use
In some cases, you’ll need to work with an HVAC technician to deal with the musty odor your AC unit is producing.
According to Gary Cremeans, experts can clean and replace the coil, install air cleaners in the airflow and offer other professional solutions.
Deep Evaporator Coil and Condenser Coil Cleaning
A professional evaporator coil and condenser coil cleaning is more effective than a DIY cleaning. Experts use specialized products that are more thorough and reach deeper into the coils.
Over time, dirty coils can cause musty odors and restrict heat transfer, which forces your unit to work harder and lose efficiency.
Condensate Line Clearing and Algaecide Treatment
Professionals use tools like wet vacuums, air pressure and algaecide tablets to clear out clogged condensate lines and deal with algae growth. These treatments will improve flow through the line and remove musty odors.
Checking for Oversized or Undersized Systems
An HVAC technician can perform load calculations to determine if your unit is too large or small for your home. Improperly sized units can cause a host of issues, including musty odors. An expert will make recommendations as necessary.
Installing a Whole-Home Dehumidifier
Texas homes with excess humidity may benefit from installing a whole-home dehumidifier to remove musty odors. These systems are integrated into the AC system and can help maintain 50 percent humidity in your home.
How to Prevent Musty AC Smells
Change Air Filters Every 90 Days
Texas homes require more frequent air filter replacements, especially during the summer months. Aim to replace it at least every 90 days.
Schedule Biannual HVAC Tune-Ups
A professional tune-up includes coil cleaning, drain flushing and a humidity check, which can help prevent moisture and mildew buildup.
Manage Indoor Humidity
Maintaining 50 percent indoor humidity can help prevent mildew buildup. Additionally, avoid activities that increase humidity in your home and maintain proper ventilation.
Keep the Outdoor Unit Area Clean
Inspect your outdoor unit regularly and remove any debris or vegetation to promote better airflow.
What Our Customers Say
“I would recommend ABC Home & Commercial to anyone who need work done on their AC/heater. Job well done!”
— Patrick S.
When to Call a Professional
Call a professional if:
- You notice a musty smell or visible mildew in your AC system
- There’s a water leak in your attic or AC closet
- The AC repeatedly shuts off
- The smell worsens over time
Do Not Hesitate to Deal With Musty Odors From Your AC Unit
The most important thing to remember is that musty odors coming from your AC unit should be addressed. In Texas, the smell is usually due to a moisture issue, which can cause more problems over time. Quick action helps prevent damage that can require costly repairs, so reach out to a local HVAC professional today.
