Why Your Air Conditioner Smells Like Mildew

Nothing ruins that first blast of cold air quite like a musty, mildew-smelling breeze. If your air conditioner smells like mildew, you're not just dealing with a bad odor — you're dealing with a problem that could impact your health, your HVAC system, and your indoor air quality.

Let’s break down the top reasons your air conditioner smells like mildew, what it means, and how to fix it before it turns into something worse.

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1. Why Does My Air Conditioner Smell Like Mildew?

That mildew smell is a tell-tale sign of moisture buildup somewhere inside your AC system. In British Columbia, where humidity can fluctuate wildly depending on the season, it’s not uncommon for ACs to develop mold or mildew when maintenance is skipped.

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Here are the most common causes when your air conditioner smells like mildew:

• Clogged Condensate Drain LineYour AC removes moisture from the air as it cools your home. That water drains away via the condensate line. When the line is blocked with debris or algae, it causes water to back up and sit in the unit—creating the perfect breeding ground for mildew. That’s often why your air conditioner smells like mildew.
• Dirty Air FiltersWhen filters are clogged with dust, pollen, or pet dander, airflow is restricted. This can lead to condensation buildup and mold formation on the evaporator coils—another reason your air conditioner smells like mildew.
• Mold Inside Ducts or VentsIf your ductwork is old, leaky, or located in humid areas like attics or crawlspaces, mold and mildew can take hold. Every time your AC runs, that smell gets pushed through your vents. If your air conditioner smells like mildew, the ducts could be the culprit.
• Frozen Evaporator CoilsIf the coils inside your unit freeze (often due to dirty filters or low refrigerant), moisture can pool and lead to that unpleasant odor once it starts melting. That’s one more way your air conditioner smells like mildew.

Book an AC inspection today to find the source

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2. What Does a Mildew Smell From the AC Actually Mean?

A musty smell isn’t just a nuisance. If your air conditioner smells like mildew, it’s a sign your system is harboring excess moisture, bacteria, or even mold. That has real implications:

• Health risks: Mold spores can trigger allergies, asthma, or respiratory issues.
• System inefficiency: A moldy or clogged system has to work harder, driving up energy costs.
• Damage risk: Unchecked moisture can damage insulation, drywall, and electrical components.

See why Eco Pro Heating is BC’s trusted HVAC provider

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3. 5 Common Reasons Your AC Smells Like Mildew

1. Full Drain Pan
Your AC drain pan collects water pulled from the air. If it fills up (often due to a clogged drain line), stagnant water breeds bacteria and mold. That’s why your air conditioner smells like mildew.

Solution: Empty and clean the pan regularly, and clear the drain line.

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2. Moldy Air Ducts
Ducts that run through damp or unsealed spaces can harbor mold. When the air flows through, it brings the smell with it, and your air conditioner smells like mildew every time it turns on.

Solution: Schedule professional duct cleaning, especially if your home smells musty year-round.

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3. Blocked Condensate Line
A blocked condensate line causes water to pool inside your system. This is one of the top culprits behind why your air conditioner smells like mildew.

Solution: Flush with vinegar every few months, or ask a pro to clear it.

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4. Dirty Evaporator Coils
Dust and bacteria on your coils create a musty odor. Worse, it reduces cooling efficiency. It’s one of the main reasons your air conditioner smells like mildew even when everything else seems fine.

Solution: Clean coils at least once a year or as part of seasonal AC service.

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5. Oversized or Undersized AC Unit
If your system isn’t sized properly for your space, it may not run long enough to dehumidify the air—or may run too long, causing condensation to linger. Either way, your air conditioner smells like mildew.

Solution: Have a professional perform a load calculation to make sure your AC is the right size for your home.

Ask us about sizing or upgrading your AC

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4. How to Get Rid of Mildew Smell From AC (DIY Guide)

If your air conditioner smells like mildew, here are some steps you can take right now:

Still noticing mildew smells? Call Eco Pro Heating

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5. When to Call a Professional

Sometimes the smell is just a dirty filter. But when it’s not, the issue often runs deeper than a quick DIY fix.

Call an HVAC technician if:

• The air conditioner smells like mildew returns shortly after cleaning
• You notice visible mold inside vents
• Your system freezes or shuts off repeatedly
• You see standing water or rust around your unit

A pro can sanitize components, clear drain lines, check refrigerant levels, and perform coil cleanings safely.

Schedule professional AC maintenance now

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6. How to Prevent Mildew Smells Long-Term

• Change filters regularly (monthly in summer) so your air conditioner smells like mildew less often
• Schedule seasonal tune-ups (spring and fall)
• Install a UV light system to kill mold spores in your air handler
• Improve insulation to avoid moisture intrusion in ducts

Preventing mildew isn’t just about eliminating odors—it’s about protecting your home and your family’s health.

View Eco Pro's HVAC services

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Frequently Asked Questions

Why does my air conditioner smell like mildew every time I turn it on?
That musty smell usually means moisture is trapped inside your AC system. It could be from a clogged condensate line, dirty filters, moldy ductwork, or frozen evaporator coils—all of which create the ideal environment for mildew to grow.

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Is it dangerous if my air conditioner smells like mildew?
It can be. A mildew smell may indicate mold or bacteria growth inside your HVAC system. This can affect indoor air quality and trigger allergies, asthma, or respiratory problems—especially in kids or seniors.

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How do I get rid of the mildew smell in my AC?
Start with these quick fixes:

• Replace the air filter
• Clean the condensate line with vinegar
• Wipe down vents and registers
• Dehumidify your space
• Run your system in fan-only mode after use

If the smell returns, call a professional to inspect your coils, ducts, and drain system.

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Why does my air conditioner smell like mildew even after changing the filter?
If the smell lingers, it’s likely caused by mold or bacteria deeper inside the system—such as in the evaporator coils, drain pan, or ductwork. At that point, professional cleaning or maintenance is recommended.

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How can I prevent my air conditioner from smelling like mildew in the future?

• Change your air filter regularly (every 1–3 months)
• Schedule spring and fall tune-ups
• Keep your home’s humidity below 50%
• Use UV lights in the air handler to kill bacteria and mold
• Seal and insulate ducts in damp areas like attics or crawlspaces

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Final Thoughts

If your air conditioner smells like mildew, don’t ignore it. That smell is a symptom—not just of a dirty system, but of deeper moisture problems that could be impacting your health and energy bills.

Regular maintenance, smart DIY care, and prompt professional help will keep your AC clean, fresh, and fully functional through every BC summer.

Contact Eco Pro Heating for expert AC care today