Why Your Air Conditioner Smells Musty (And How To Fix It)
- Brett Wells
- 5 days ago
- 3 min read
A musty smell coming from your air conditioner is one of the most common complaints from homeowners in coastal climates.
If you notice a damp or mouldy smell when your AC turns on, it usually means moisture has built up somewhere inside the system.
This problem is particularly common in humid coastal cities like the Gold Coast, where warm ocean air carries high levels of moisture throughout the year.
Understanding what causes these smells can help you solve the problem quickly and prevent more serious issues like mould growth or poor indoor air quality.
The Most Common Cause: Moisture Inside The System
Air conditioners naturally remove moisture from the air while cooling.
When warm air passes over the cold evaporator coil, condensation forms and drains away through the system.
However, if moisture cannot drain properly, it can build up inside the unit and create the perfect environment for mould and bacteria.
This trapped moisture is usually what produces that distinct musty odour.
Common Reasons Your AC Smells Musty
Dirty Air Filters
Clogged filters restrict airflow and allow moisture to accumulate inside the unit.
Dust trapped in the filter can also become damp and start to smell over time.
Most filters should be cleaned or replaced every 1–3 months, depending on usage.
Blocked Condensate Drain
The condensate drain removes water produced during cooling.
If this drain becomes blocked by debris, mould or algae, water can pool inside the unit.
Standing water inside the system often leads to musty or damp smells.
Mould Inside The Evaporator Coil
The evaporator coil is cold and often slightly damp, which makes it a common place for mould to develop.
When the system turns on, air blows across the coil and spreads the smell throughout your home.
Professional cleaning is usually required to remove mould from internal components safely.
Dirty Ductwork
In ducted systems, moisture inside ducts can allow mould to develop.
When the system starts, the smell travels through vents into different rooms.
Duct cleaning may be required if contamination is present.
Why You Shouldn’t Ignore AC Smells
Musty air conditioner smells aren’t just unpleasant. They can also indicate problems affecting your health and home.
Mould spores circulating through your home can trigger:
• allergies
• asthma symptoms
• respiratory irritation
• headaches
Long-term moisture problems can also damage ceilings, walls and insulation.
Industry organisations such as AIRAH recommend addressing indoor air quality issues promptly to prevent larger system failures.
How To Prevent Musty Air Conditioner Smells
Fortunately, most causes of AC odours are preventable with regular maintenance.
Clean Filters Regularly
A clean filter improves airflow and prevents dust buildup inside the system.
Run The System Regularly
Leaving an AC unit unused for long periods can allow moisture to sit inside the system.
Running it occasionally helps keep components dry.
Use “Dry Mode” During Humid Weather
Most modern systems include a dehumidification mode that helps remove excess moisture from indoor air.
Schedule Professional Servicing
Annual servicing ensures coils, drains and internal components remain clean and functioning properly.
When To Call A Professional
If the smell continues after cleaning filters and adjusting settings, the problem may be inside the system.
A technician can inspect:
• evaporator coils
• condensate drain lines
• internal mould growth
• airflow issues
Professional servicing can restore fresh airflow and prevent larger mechanical problems.
Final Thoughts
Musty air conditioner smells are usually caused by moisture buildup and mould inside the system.
By maintaining your unit and addressing issues early, you can protect your home’s air quality and ensure your system runs efficiently.
If your AC smells musty when it turns on, a professional inspection can quickly identify and fix the problem.
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