Allergy season in Idaho isn’t fun, especially when it’s exacerbated by musty and moldy smells when your air conditioner kicks on. If your air conditioner smells musty without an obvious cause, read on to find out the most common underlying causes, homeowner troubleshooting tips, and preventative strategies.

What Causes a Musty AC Smell?

Musty AC smells can pose several hazards to your household’s well-being and your home’s structure. Musty smells typically stem from mold and often correlate with allergic reactions, respiratory infections, and other health concerns. Musty smells also suggest leaks or damage to your home, which can escalate without the attention of a licensed HVAC technician.

See the main causes of musty smells below.

1.Mold and mildew

A musty AC smell typically results from mold or mildew in your AC system. Many ACs briefly smell musty for a few cycles after a winter of disuse, but a persistent odor indicates microbial growth in your system. AC systems regulate the humidity in your home, and trapped moisture — in combination with bacteria — is the perfect home for a new patch of mold.

2. Ductwork damage

Poor insulation and sealing, uncommonly high humidity, roofing leaks, and other factors can result in condensation buildup in your ductwork. This excess moisture becomes a habitat for mold, and air can carry its spores and musty smell throughout your AC system and home. Ductwork damage often comes with strange AC sounds, such as whistling or rattling.

3. Dirty AC coils

Dirt, dust, and other particles spread through your system and can slowly build up on your evaporator coil. Excessive dirt may cause the coil to freeze, trapping bacteria and humidity within the ice. Air to your supply vents carries the resulting mold and mildew spores and smells.

4. Dirty AC filters

Air filters require replacement about every three months — potentially less, depending on your home. If filters accumulate excessive dirt and debris, more debris and bacterial particles can travel throughout your system to land elsewhere, such as on the evaporator coil. Mold can develop on the air filter itself and infuse the airflow with a musty scent.

5. Clogged AC drain

AC systems feature a drain line pipe that removes excess moisture from the inside and transfers it to a drain pan. Regular wear and tear, poor insulation, or other issues can result in damage to the drain line and pan. A clogged line prevents water from evaporating outside and creates a backup of humidity.

6. Improper AC size

A larger AC isn’t necessarily better. AC units exceeding your home’s needs may cool your rooms too quickly. This prevents the unit from absorbing adequate humidity, resulting in frequent and short cycles.

Troubleshooting Tips for Idaho Homeowners

Mold and mildew can develop in your AC system for several reasons. Here are some underlying causes and troubleshooting tips for moldy and musty AC smells in your home.


Musty smells from air conditioners suggest various underlying causes, depending on the source. Consider where the musty smell is coming from or where it’s strongest. If it’s strongest from the indoor unit, the evaporator coil or drip pan may have mold. Musty odors directly from the vents or throughout the home during cycles can indicate mold in the AC ductwork or dirty air filters.


Weather fluctuations can contribute to mold growth in your AC system in several ways. While Idaho isn’t especially humid, a relative shift to high humidity may increase the moisture in your system beyond its usual load, creating ideal conditions for mold growth.

Condensation may occur when warm outdoor air meets evaporator coils or other cool surfaces. Mold may go undetected during the winter when your AC isn’t in use, and warm temperatures in the summer can accelerate its growth.

Recent changes

Seemingly minor events sometimes have big effects on your air conditioning. For example, power outages can temporarily restrict air circulation and cause moisture accumulation. This environment may last long enough to accelerate mold growth in the system.

Water leaks, such as from damaged pipes or ceilings, also increase moisture levels in the system. Home renovations can increase pollutants in your home’s air and strain the AC’s filtration, potentially resulting in blockages.

Preventing Future Musty Smells

Proactive strategies and regular professional maintenance are the best ways to keep your system mold-free. Try these tips for preventing musty AC smells:

  • Change air filter: Depending on your home’s needs, you should replace your air filter at least every three months or more.
  • Open vents: Avoid closing vents or obstructing them with furniture or other items. Closing vents can disrupt the system’s pressure and airflow and cause it to overwork.
  • Repair ductwork: Inspect your ductwork for condensation, leaks, and mold. If you find any issues, schedule repair and cleaning services promptly.
  • Dehumidifier: Use a dehumidifier to assist your AC with regulating humidity in your home.
  • Clean drain line: To avoid stagnant water in your AC, inspect drain lines and pans for damage or clogs and clean or schedule repairs as needed.
  • Professional maintenance: Routine maintenance from a licensed HVAC technician includes thorough inspections, cleaning, and adjustments to keep your system at optimal performance. They can also locate underlying concerns or risk factors for mold.

Enjoy Clean and Cool Air With Express Plumbing AC Services in Idaho

Persistent musty AC smells can indicate underlying system problems and typically require prompt repair or AC maintenance.

The team at Express Plumbing, Heating, and Air includes experienced, knowledgeable, and licensed HVAC technicians who can address musty AC smell concerns. We offer expert HVAC service, excellent customer service, and cost-effective pricing. Contact us today to schedule a service.

Frequently Asked Questions

Is AC mold toxic?

Mold spores can carry toxicity levels high enough to worsen or cause asthma and respiratory infections.

How common is mold in AC?

Mold is relatively common in Idaho AC systems, but it can go undetected for long periods of time, such as over the winter when the air conditioner is in disuse.

Brad Jordan

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