AC Smells Mouldy: Causes and How to Treat It − Follow Our Easy Guide

Have you ever turned on your air conditioner and been hit with a musty, mouldy smell? It’s not just unpleasant − it could be a sign of a bigger problem.

Mould and mildew thrive in damp, dark places, and your AC unit provides the perfect conditions if moisture gets trapped inside. Ignoring the issue won’t just make your home smell bad; it can also affect your indoor air quality and even trigger allergies or respiratory issues.

The good news? You don’t have to put up with it. Understanding what’s causing the smell is the first step to getting rid of it for good.

In this blog, we’ll break down the common reasons your air conditioner smells mouldy and give you practical solutions to freshen up your system and keep your air clean.

What Does Mould Smell Like?

If your air conditioner gives off a musty, damp, or earthy smell when you turn it on, chances are mould or mildew is growing inside. The scent is often compared to old, wet socks, rotting wood or a damp basement.

It’s an unmistakable, stale odour that lingers in the air and can quickly spread throughout your home.

Mould thrives in moist environments, and your AC unit can create the perfect breeding ground if it’s not properly maintained. The smell comes from tiny mould spores releasing into the air, which isn’t just unpleasant − it can also affect your health. Prolonged exposure may cause allergy symptoms, respiratory issues or worsen asthma.

If you notice this smell, it’s important to act fast. The longer mould is left untreated, the worse it gets. In the next sections, we’ll explore why your AC smells mouldy and how to fix it.

Common Causes of a Mouldy Smell in Your AC

If your air conditioner smells musty or mouldy, it’s likely due to excess moisture and poor airflow. Here are the most common culprits:

• Mould and mildew growth – Your AC creates a cool, damp environment – ideal for mould and mildew. These fungi often grow on the evaporator coils, drain pan or inside the ductwork, releasing that stale, musty odour when the system runs.
• Clogged or dirty air filters – Air filters trap dust, dirt and moisture. If they’re not cleaned or replaced regularly, they become a breeding ground for mould spores, which then circulate through your home.
• Blocked condensate drain line – Your AC’s drain line removes moisture, but if it’s clogged, stagnant water can build up inside the unit, leading to mould growth and an unpleasant smell.
• Damp or leaky ductwork – If your ductwork has leaks or poor insulation, condensation can form inside, creating the perfect conditions for mould to develop and spread.
• Standing water in the system – A full or dirty drain pan can collect water, allowing bacteria and mould to grow over time.

Addressing these odour issues quickly and efficiently will help eliminate the smell and prevent mould from coming back.

How to Eliminate Mouldy Smells from Your AC

If your air conditioner smells mouldy, don’t worry – you can take several steps to eliminate the odour and keep your system fresh. Here’s what to do:

• Replace or clean the air filter – A dirty filter traps moisture and mould spores. Replace disposable filters every 1-3 months or clean reusable ones with warm, soapy water.
• Clean the evaporator coils – Dust and moisture buildup on the coils create the perfect mould breeding ground. Use a coil cleaner or a mild vinegar solution to remove grime and bacteria.
• Flush the condensate drain line – A clogged drain line can cause stagnant water to sit inside your unit. Pour a mixture of white vinegar and warm water into the drain to clear any blockages.
• Wipe down vents and ducts – If mould has spread into your vents, wipe them down with a mould-killing solution. If the smell persists, professional duct cleaning may be needed.
• Use an anti-mould spray – Spraying a non-toxic, mould-resistant treatment inside your unit can help prevent future growth.
• Improve ventilation and reduce humidity – Use a dehumidifier and keep doors and windows open occasionally to prevent excess moisture buildup.

These steps will help remove mouldy odours and keep your heating, ventilation and air conditioning (HVAC) system smelling fresh.

How to Prevent Mould Growing in Your AC

Preventing mould in your air conditioner starts with keeping it clean and dry. Since mould thrives in moisture, the key is reducing damp conditions inside your unit.

1. First, clean or replace your air filters regularly. A dirty filter traps dust and moisture, creating the perfect breeding ground for mould. Most filters should be replaced every 1-3 months, depending on usage.
2. Next, check the condensate drain line. This small pipe removes excess moisture, but if it gets clogged, water can back up and encourage mould growth. Flush it with a mixture of vinegar and water every few months to keep it clear.
3. Keeping humidity levels low in your home also helps. If your space is naturally humid, consider using a dehumidifier to prevent excess moisture buildup.
4. Finally, schedule routine AC maintenance. A professional can inspect your system, clean internal components, and ensure proper airflow.

By taking these simple steps, you’ll keep your HVAC mould-free, improve air quality and enjoy fresh, clean air all year round.

When to Call a Professional

Sometimes, a mouldy-smelling air conditioner needs more than just a quick clean − you may need professional help. If you’ve replaced the filter, cleaned the coils, and flushed the drain line but the musty smell persists, it’s a sign that mould has spread deeper into the system.

One major red flag is visible mould growth inside the unit or ductwork. If you notice black, green, or white patches on the vents or AC components, professional cleaning is the safest way to remove it completely. Mould spores can easily spread, and improper cleaning can make the problem worse.

Another reason to call an expert is poor airflow or excess humidity. If your AC isn’t properly dehumidifying your home, moisture will continue to build up, creating ideal conditions for mould. A qualified HVAC technician can diagnose airflow issues, repair leaks and improve overall system performance.

Regular professional maintenance can also prevent future problems. An HVAC specialist will clean hard-to-reach areas, check for leaks, and ensure your system runs efficiently − keeping your home mould-free and your air fresh.

Clean Your AC and Breathe Easy

A mouldy-smelling air conditioner isn’t just unpleasant − it’s a sign of excess moisture and potential mould growth inside your system.

Ignoring the issue can lead to poor indoor air quality and even health problems like allergies and respiratory irritation. Fortunately, with a few simple steps, you can eliminate the smell and keep your AC fresh.

Regular maintenance, like cleaning or replacing filters, flushing the drain line and keeping humidity levels in check, can go a long way in preventing mould. If the musty smell persists despite your efforts, it may be time to call a professional for a thorough cleaning or inspection.

By taking action early, you can enjoy clean, fresh air and keep your air conditioner running efficiently for years to come. Don’t let mould take over − treat the issue today.

Please note: This information is provided for advice purposes only. Regulations differ from state to state, so please consult your local authorities or an industry professional before proceeding with any work. See our Terms & Conditions here.