Why Does My AC Smell Musty When It Turns On?

If you notice a musty smell when your AC turns on, you're not alone. This is a common complaint among homeowners in Hillsboro, Beaverton, Tigard, and other areas around Portland—especially during the first hot days of the season. While it may seem like just a minor annoyance, that smell could be a sign of a deeper issue within your air conditioning system.

At Aviator Heating & Cooling, we help homeowners diagnose and fix these odor problems quickly and professionally. Let’s break down what causes that musty smell and what you can do to fix it.

Common causes of musty AC smells

If your AC smells musty when it turns on, the issue is often caused by moisture or mold buildup inside your system. Below are the most common culprits and how they lead to musty odors.

1. Mold or mildew in the evaporator coil

The evaporator coil is a common source of musty smells, especially if it hasn’t been cleaned in a while.

What to look for:

• • • Musty odor starts immediately when AC turns on
    • Cooling performance seems weaker than usual
    • System hasn’t been serviced or cleaned in over a year
    • AC is located in a damp area like a basement or crawl space

How we help: We offer evaporator coil cleaning as part of our professional AC maintenance services to remove mold and restore airflow.

2. Clogged or dirty condensate drain line

A blocked drain line causes water to back up into the system, leading to odors and potential leaks.

Signs of a clogged line:

• • • AC smells musty or “wet” after a few minutes of operation
    • Water pooling near the indoor unit or drain pan
    • Visible algae, mold, or slime near the drain line
    • System may shut off due to overflow protection

Our solution: We flush and clear clogged drain lines, inspect for blockages, and ensure proper drainage during every maintenance visit.

3. Dirty or moisture-saturated air filters

When filters become too dirty—or absorb too much moisture—they can promote mold growth directly in the airflow path.

Warning signs:

• • • Filter appears damp, discolored, or smells musty
    • Airflow is weak or uneven in certain rooms
    • Dust levels in your home are unusually high
    • System cycles more frequently to maintain temperature

Pro tip: We recommend replacing your air filter every 1–3 months depending on usage and household conditions.

4. Mold inside your ductwork

Mold in the ductwork can spread airborne spores and odors throughout your home, impacting both comfort and health.

Indicators of mold in ducts:

• • • Smell is strongest from one or more supply vents
    • Family members experience allergies or asthma flare-ups
    • Ducts are older or were never professionally cleaned
    • Home has high humidity or poor attic/basement ventilation

What we offer: Aviator Heating & Cooling provides duct inspections, cleaning, and sealing services to stop mold at the source.

5. Standing water in the AC drain pan

A damaged or misaligned drain pan can trap moisture, becoming a breeding ground for mildew.

Drain pan red flags:

• • • Odor is strongest after the AC has been running for a while
    • Visible standing water beneath the indoor unit
    • Dripping or rusting near the base of the unit
    • System is leaking or shutting down unexpectedly

Fix it fast: During our AC maintenance visits, we inspect the drain pan for cracks, clogs, and proper drainage—preventing odor and water damage.

Is the smell dangerous?

A musty smell from your AC isn’t just unpleasant—it could be a sign of a deeper issue that may impact your health. While the odor itself isn’t harmful, the underlying cause often is—especially if it involves mold, mildew, or bacterial growth inside your HVAC system.

Why musty smells should never be ignored:

• • Mold spores are the most common culprit behind musty odors. When inhaled, they can cause or aggravate health issues, especially for those with existing respiratory conditions.
  • Allergies and asthma can be worsened by poor indoor air quality. Mold, dust, and other airborne particles that circulate through your vents can trigger coughing, sneezing, eye irritation, or shortness of breath.
  • Young children, seniors, and immunocompromised individuals are particularly vulnerable to indoor pollutants. Even mild exposure to mold spores can result in prolonged symptoms or discomfort.

If the smell is strongest when the system turns on—or if you’ve noticed increased allergy symptoms, chronic congestion, or a damp, heavy feeling in the air—it’s time to take action.

What starts as an odor can become a bigger problem

Musty smells are often a warning sign that something in your system is retaining moisture. If ignored, this can lead to:

• • Mold spreading into ductwork or living spaces
  • Moisture damage to insulation, drywall, or flooring
  • Increased system strain, reducing energy efficiency
  • Higher repair costs down the line due to long-term damage

The good news? In most cases, these issues can be caught early and resolved quickly with a professional inspection and cleaning.

At Aviator Heating & Cooling, we’ve helped many homeowners in Hillsboro, Forest Grove, and Beaverton restore clean, healthy indoor air with our AC maintenance and air quality services. If you suspect your system is affecting your home’s air quality, we’ll help you get to the bottom of it—fast.

How to get rid of a musty AC smell

If your air conditioner smells musty, don’t just cover it up with air fresheners or scented candles. The key is to eliminate the source of the odor, not just mask the symptoms. A musty smell is a warning sign—usually of mold, mildew, or stagnant moisture inside your HVAC system.

Here’s how to get rid of it the right way:

Step 1: Schedule a professional HVAC inspection

The first step is to pinpoint where the odor is coming from. A trained technician from Aviator Heating & Cooling can perform a full inspection of your system, including:

• • • Checking the evaporator coil for mold or mildew
    • Inspecting the drain pan and condensate line for clogs or water buildup
    • Examining your air filters for contamination or excessive dirt
    • Assessing your ductwork for moisture or microbial growth
    • Testing airflow and humidity levels to identify ventilation issues

This step ensures the real cause of the smell is found and not overlooked.

Step 2: Deep clean the system

Once the source is identified, a thorough cleaning is often the fastest and most cost-effective way to resolve the issue.

Aviator Heating & Cooling offers comprehensive AC cleaning and maintenance services, including:

• • • Evaporator coil cleaning to remove mold and mildew buildup
    • Condensate drain line flushing to clear blockages and ensure proper drainage
    • Air filter replacement to improve airflow and remove contaminants
    • Drain pan cleaning and alignment to prevent standing water
    • Basic duct cleaning if accessible and appropriate

Regular cleanings not only eliminate odors but also extend the life of your system and improve cooling efficiency.

Step 3: Repair or replace worn-out components

If your system is older or poorly maintained, there may be worn or malfunctioning parts contributing to moisture problems.

We’ll evaluate your AC system and recommend solutions such as:

• • • Replacing a cracked or corroded drain pan
    • Upgrading an aging thermostat or blower motor
    • Installing a new, energy-efficient AC unit if yours is near the end of its life
    • Switching to a ductless mini-split for better humidity control and zone comfort

We’ll never upsell you something you don’t need. Our goal is to solve your comfort problems with the most practical and effective solution.

Step 4: Improve your home’s air quality

Once the immediate odor problem is fixed, you can take steps to prevent it from coming back by improving your indoor air quality.

Aviator Heating & Cooling can help with:

• • • Whole-home air filtration systems to capture allergens and mold spores
    • UV air purifiers to neutralize bacteria, mold, and odors at the source
    • Dehumidifier integration to keep moisture levels in check during humid Oregon summers
    • Smart thermostats to monitor and maintain ideal indoor conditions

These upgrades not only keep your AC smelling clean—they also make your home healthier and more comfortable year-round.

Preventing future AC odors

To keep your AC smelling fresh and running efficiently, we recommend:

• • Changing your air filter every 1–3 months
  • Scheduling seasonal AC maintenance in spring
  • Keeping your home’s humidity levels under control
  • Ensuring your HVAC system drains properly
  • Having your ductwork inspected periodically

Regular maintenance isn’t just about performance—it’s also about health and comfort. Our Aviator Club Membership includes annual maintenance visits, priority service, and peace of mind all year round.

Proudly serving Hillsboro and surrounding areas

Aviator Heating & Cooling provides expert AC repair, maintenance, and installation services throughout the Portland Metro area, including:

• • Hillsboro
  • Beaverton
  • Tigard
  • Forest Grove
  • Aloha
  • Lake Oswego
  • North Plains
  • Sherwood
  • Portland and nearby communities

Call today to eliminate musty AC smells for good

If your air conditioner smells musty when it turns on, don’t ignore it—schedule a professional inspection with Aviator Heating & Cooling. We’ll pinpoint the issue, provide lasting solutions, and restore fresh, clean air to your home.

Call now or request an estimate online. We proudly serve Hillsboro and the greater Portland area with honest, professional HVAC service you can rely on. Whether it’s AC repair, a system upgrade, or air quality improvements, we’re here to help you breathe easier.