Your AC Smells Mouldy? Causes and Solutions

If your air conditioner smells mouldy every time you turn it on, you’re not alone − and you’re definitely not imagining it.

That musty odour usually means there’s mould or mildew growing somewhere in your system, and it’s not just unpleasant − it could also affect your health. Breathing in mould spores can lead to headaches, allergies, or even respiratory issues over time.

The good news is a mouldy smell doesn’t mean your air conditioner is doomed. With the right know-how and a little maintenance, you can clear the air and stop the smell from coming back.

In this blog, we’ll break down what causes that musty scent, how to get rid of it, and what you can do to keep your AC fresh, clean and healthy all year round.

Why Does Your AC Smell Mouldy?

That musty, damp smell coming from your heating, ventilation and air conditioning (HVAC) system is usually a sign that mould or mildew has started to grow inside the unit. Why does this happen? It all comes down to moisture.

Air conditioners work by pulling warm air from your home, cooling it down and removing humidity in the process. But if moisture doesn’t drain properly or parts of the system stay damp for too long, it creates the perfect environment for mould to grow.

The most common spots for mould buildup include the air filter, evaporator coils, and drip tray. If these components are dirty or clogged, airflow gets restricted and moisture lingers − giving mould spores a place to thrive. Over time, this leads to that all-too-familiar mouldy smell blowing through your vents.

It’s not just about the smell, either. Mould can circulate through the air you breathe, triggering allergies or respiratory issues. That’s why it’s important to understand the cause and take action early before the issue gets worse.

Common Causes of Mouldy AC Smells

If your air conditioner smells like old gym socks or damp laundry, mould is probably to blame. Here are the most common reasons that funky smell is creeping through your air conditioning vents:

• Dirty air filters: When filters get clogged with dust and debris, they trap moisture − the perfect breeding ground for mould and mildew.
• Blocked or overflowing drain pan: The drain pan collects condensation from your HVAC system. If it’s full, blocked, or not draining properly, water sits inside the unit and encourages mould growth.
• Clogged condensate drain line: This little pipe carries excess moisture away from your AC. If it’s blocked, water backs up and increases humidity inside the unit.
• Moisture in ductwork: Leaky ducts or high humidity in your home can allow moisture to build up inside your ductwork − leading to mouldy smells every time the system runs.
• Mould on coils or interior components: If coils or internal parts get dirty and damp, they become a hotspot for mould. This often requires a professional deep clean to fix.

How to Get Rid of the Mouldy Smell

Getting rid of that musty smell in your air conditioner isn’t just about freshening the air − it’s about tackling the mould at its source.

Here are some simple but effective steps you can take:

• Clean or replace the air filter: A dirty filter is a common culprit. Remove it, give it a clean (if reusable), or replace it entirely. This helps improve airflow and remove trapped moisture and mould spores.
• Empty and clean the drain pan: Check the drain pan under your unit for standing water. Give it a good scrub with soapy water or a mould-killing solution.
• Clear the drain line: Use a wet/dry vacuum or a pipe cleaner to unclog the condensate drain line and prevent future water build-up.
• Clean the coils: Dirty coils can harbour mould. Gently clean them using a coil cleaner or warm water and mild detergent.
• Run the fan mode: After using the AC, switch to fan-only mode for a while to help dry out moisture that encourages mould growth.

How to Prevent Mould from Returning

Once you’ve gotten rid of the mouldy smell in your air conditioner, the next step is making sure it doesn’t come back. Here’s how to keep things fresh and mould-free long term:

• Change or clean the filter regularly: Don’t wait for a smell to start − clean reusable filters monthly or replace disposable ones every 1–3 months, depending on usage.
• Keep the area around your AC dry: Check for leaks or excess condensation around your indoor unit. Wipe up moisture and make sure the space has good airflow.
• Clear the drain line every few months: Use a mixture of vinegar and warm water to flush the line and prevent clogs and standing water.
• Run the fan after cooling: After turning off the cooling function, switch your system to fan mode for 10-15 minutes. This helps dry out any leftover moisture.
• Schedule regular maintenance: Having a professional service your AC at least once a year can catch early signs of mould and keep your system running smoothly.

When to Call a Professional

Sometimes, a mouldy air conditioner is more than a simple DIY fix. If you’ve cleaned the filters, checked the drain lines and the musty smell still lingers, it might be time to call in a professional.

A persistent odour can be a sign of mould deep within the ductwork, coils, or other internal components − areas that are hard to reach without the right tools and expertise.

You should also contact a technician if you notice visible mould inside the unit, if your air conditioner is leaking water, or if anyone in your household is experiencing allergy symptoms or breathing problems that worsen when the AC is on. These could indicate a larger issue that needs professional treatment.

An HVAC technician can carry out a deep clean, inspect for hidden problems, and make sure your system is running safely and efficiently.

Getting expert help not only clears up the mould but can also prevent future build-up, saving you money and keeping your indoor air fresh and healthy. If something more serious is necessary, such as AC replacement, a professional can give you valuable advice.

Mouldy Odour is a Warning

A mouldy-smelling air conditioner is more than just an annoyance − it’s a sign that something’s not quite right inside your system.

Whether it’s trapped moisture, a clogged filter or mould growing on the coils, it’s important to tackle the issue sooner rather than later. Left unchecked, mould can affect your air quality and even your health.

However, most causes are easy to fix with a bit of cleaning and regular maintenance. And if the smell sticks around, a professional can help get to the root of the problem.

With a little attention and care, your AC can go back to doing what it does best − keeping you cool, comfortable and breathing fresh, clean air all year round.

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.