Lion City Aircon

The Real Reasons Your Aircon Gets Moldy and How to Stop It for Good

If you’ve switched on your aircon and been hit with a musty, sour smell, or if you’ve spotted dark spots on the vents, your unit has mould. It’s one of the most common aircon problems in Singapore, and one of the most preventable. After 22,000+ jobs since 2016, we’ve cleaned mould out of more aircons than we can count. HDB flats, condos, landed homes, offices. The patterns are consistent, the causes are well understood, and the fix is straightforward once you know what’s actually going on.

This guide explains why mould keeps coming back, what causes it specifically in Singapore’s climate, the warning signs you shouldn’t ignore, and exactly what to do about it. From DIY prevention to when you need professional aircon servicing.

Why Aircon Mould Is Especially Bad in Singapore

Mould thrives in three conditions: warmth, moisture, and organic material to feed on. Singapore gives it all three, year round. Where other countries see seasonal mould problems, we see chronic ones.

1. 80%+ humidity, every day of the year

Singapore’s relative humidity rarely drops below 75%, and most days sits between 80 and 95%. That moisture condenses inside your aircon’s evaporator coil constantly. Every time the unit runs, water collects on the coils, in the drain pan, and along the drainage pipe. If that water doesn’t drain away properly, or if dust traps it inside, mould has the perfect breeding ground.

2. Dust and bacteria from urban density

HDB flats sit close together. Construction is constant. Outdoor units share airflow with neighbours’ condensers. All this means your aircon pulls in dust, skin cells, pet dander, cooking residue, and bacteria every minute it’s running. These organic particles are mould food. They settle on damp internal parts and start feeding fungal growth within days.

3. 24/7 cooling habits

Most Singapore households run aircon nightly, and many run it during work hours too. Each cycle of cooling produces condensation. Each switch off leaves the inside damp. Compared to the seasonal use you’d see in Europe or America, your aircon never gets a proper dry out period.

The Real Causes of Aircon Mould (What’s Actually Happening Inside)

Most homeowners think mould comes from dirty filters. That’s part of it. But the real problem is usually deeper inside the unit.

A blocked drainage pipe (the #1 cause)

This is what we find in roughly 70% of mouldy aircon jobs. Condensation drains out through a small pipe behind your fan coil unit. Over time, dust, slime, and “aircon jelly” build up inside the pipe and block it. Water then sits stagnant in the drain pan, and within weeks you’ve got a thriving mould colony. Vacuuming the drainage pipe quarterly is the single most effective mould prevention step.

A dirty blower wheel

The blower wheel is the spinning fan that pushes cooled air into your room. It collects an enormous amount of dust because it’s always wet (from condensation) and always spinning through dirty air. That dust holds moisture against the wheel, which then becomes a black mould layer that gets visibly thicker every month. You can’t see this from outside the aircon. It’s only visible during a proper service when the cover comes off.

Coil and fin contamination

The evaporator coil (the cold metal grid behind the filter) gets coated in a sticky biofilm over time. This biofilm is partly mould, partly bacteria, partly mineral residue. Regular servicing wipes it off with proper tools and chemicals. A chemical wash removes it completely. DIY filter cleaning doesn’t touch it.

Stagnant water in the drain pan

If the drainage pipe is partially blocked, water sits in the pan. If the unit isn’t sloped correctly (a common HDB installation issue), water pools at the back where you can’t see it. Either way: stagnant water plus warm temperature plus organic dust equals mould factory.

Irregular servicing

If you only service when the aircon stops cooling, you’re already 6 to 12 months past the point where mould started forming. Quarterly servicing is the Singapore baseline, and it exists primarily to prevent this.

Warning Signs Your Aircon Already Has Mould

You don’t always see mould directly, but your body and your aircon will give you signals:

Musty, sour, or “old wet towel” smell when the aircon switches on. Strongest in the first 30 seconds before the airflow stabilises.
Black, green, or grey spots on the vents, blower flap, or air inlet.
Visible slime or “goo” around the drainage outlet or behind the front cover.
Increased sneezing, coughing, or eye irritation when in the aircon cooled room, especially first thing in the morning.
Asthma or allergy symptoms getting worse in your home but better outdoors.
Water dripping from the front of the unit (clogged drainage equals mould friendly conditions).
Weak or uneven airflow. Biofilm on the coils is reducing efficiency.
A pet that suddenly avoids the room. Dogs and cats smell mould long before humans do.

If you’ve noticed any 2 or more of these, your aircon needs cleaning urgently. WhatsApp us +65 8818 5781 for a same day inspection.

The Health Impact (Why This Isn’t Just About Smell)

Aircon mould isn’t a cosmetic problem. The spores released into your air every time the unit runs cause real health issues, especially for vulnerable people:

Children. Developing respiratory systems are more sensitive to mould spores. Persistent coughs and increased asthma attacks are common warning signs in kids sleeping in mouldy aircon rooms.
Elderly family members. Weakened immune systems make them more susceptible to fungal infections from prolonged exposure.
People with asthma or allergies. Mould is one of the top three indoor triggers. If your asthma is worse at home than elsewhere, your aircon is likely the cause.
Anyone working from home. 8+ hours daily breathing recirculated air through a mouldy unit causes chronic sinus inflammation, fatigue, and “brain fog”.
Pets. Dogs and cats living in mould contaminated air develop respiratory infections faster than humans.

The science is clear: prolonged exposure to indoor mould (including from aircon units) is linked to bronchitis, hypersensitivity pneumonitis, and chronic sinus problems. This is why we don’t soft pedal mouldy aircon issues with customers. It genuinely matters.

How to Stop Aircon Mould. For Good.

1. Quarterly aircon servicing

The most effective single step. A proper general servicing dismantles the cover, water tray, and blower, vacuums the drainage pipe, cleans the coils with proper chemicals, and runs a full system test. This catches mould while it’s still surface level and clears the conditions that let it grow. See our servicing frequency guide for full details on intervals.

2. Chemical wash when general servicing isn’t enough

If mould is severe or recurring, a chemical wash is the next step. We use proper cleaning agents that dissolve the biofilm on the coils, kill mould spores at the root, and flush deep contamination from the drainage system. See chemical wash pricing. For really old units with deep contamination, a chemical overhaul (full dismantling and deep clean) may be needed.

3. Clean your filter every 2 to 4 weeks

This is the DIY step most homeowners can do. Pull out the filter, rinse with water, let it dry fully, reinstall. Doesn’t replace professional servicing but slows down the mould feeding dust buildup. Easy, takes 5 minutes per filter.

4. Run fan mode for 10 to 15 minutes after each use

This is the trick nobody tells you. Before switching off the aircon, leave it on FAN mode (no cooling) for 10 to 15 minutes. This dries out the internal components and prevents the damp environment mould loves. Modern inverter units sometimes have an “auto-clean” function that does this automatically. Use it.

5. Use Dry Mode during very humid weather

Dry mode (sometimes called dehumidify mode) removes moisture from the air without aggressive cooling. Useful during rainy season when humidity spikes. Lowers indoor moisture which slows mould growth throughout the home, not just the aircon.

6. Don’t seal the room completely

Open the windows for 10 to 15 minutes every morning. This isn’t about losing the cool air. It’s about letting moisture and stale air cycle out before you fire up the aircon. Singapore air is humid but fresh outdoor air is still drier than recycled indoor air with mould spores.

When You Definitely Need a Professional

DIY can prevent and slow mould growth. It can’t remove established mould. Call professional aircon servicing if:

The musty smell returns within days of last servicing
You can see visible black or green spots that don’t wipe off
Anyone in the household has worsening respiratory symptoms
The unit hasn’t been serviced in 6+ months
Water is dripping from the front (drainage pipe is blocked, mould is feeding on stagnant water)
The aircon is older than 3 to 4 years and has never had a chemical wash
You’ve cleaned the filter but the smell persists

At Lion City Aircon, we’ve cleaned mould from over 22,000 jobs across Singapore since 2016. We know what works and what doesn’t. 5.0★ Google rating with 1,500+ verified reviews.

Book Your Mould Inspection Today

If your aircon is mouldy, every day you wait means more spores in your air, more health impact, and more contamination spreading deeper into the system. Don’t wait. Same day appointments available across all of Singapore.

Call or WhatsApp: +65 8818 5781
Book Online: Booking page
Email: contact@lioncityaircon.sg

Frequently Asked Questions

Can I clean aircon mould myself?

You can clean the filter and exterior surfaces yourself. But established mould lives on the evaporator coil, inside the blower wheel, and in the drainage pipe. Places that require dismantling the cover with proper tools. DIY doesn’t reach these areas. If the musty smell is back within a week of cleaning the filter, the mould is internal and you need professional servicing.

How dangerous is aircon mould for my family?

For most healthy adults, prolonged exposure causes sinus issues, headaches, and fatigue. For children, elderly people, asthma sufferers, and pets, it’s significantly worse. It can trigger or worsen respiratory conditions. The longer the exposure, the more chronic the impact. If anyone in your household has unexplained breathing problems that get better when away from home, your aircon is a likely cause.

What’s the difference between regular servicing and a chemical wash for mould?

Regular servicing cleans surface level mould and prevents buildup. Chemical wash uses cleaning agents to dissolve deeper biofilm and kill spores at the root. Needed for severe or recurring mould. Chemical overhaul takes it further with full disassembly. For most homes with quarterly servicing, regular service is enough. For homes that haven’t been serviced in 6+ months, a chemical wash is usually needed to reset everything. See our chemical wash page.

How long after servicing should the mould smell go away?

Immediately. After a proper servicing or chemical wash, the smell should be completely gone when you switch the aircon back on. If it returns within days or weeks, either the servicing was incomplete, or the unit is contaminated deeper than expected and needs a chemical wash or overhaul.

Why does my aircon smell musty even though I just cleaned the filter?

Because the mould isn’t on the filter. It’s on the blower wheel, evaporator coil, or in the drainage pipe. Filter cleaning only removes the most superficial dust. Internal mould requires professional servicing to access.

Is the smell harmful or just unpleasant?

The smell itself isn’t directly harmful. It’s caused by gases released by the mould (called MVOCs). But the smell is a warning sign that live mould spores are being released into your air. Those spores are what cause health issues. Treat the smell as an early warning signal, not just an inconvenience.

How do I prevent mould from coming back after servicing?

Three things: quarterly servicing (don’t stretch past 3 months in Singapore), fan mode for 15 minutes before switching off (dries internal parts), and filter cleaning every 2 to 4 weeks (slows the buildup that feeds mould). Households with kids, pets, or heavy aircon use should service every 2 to 3 months instead of quarterly.

Why is my aircon mouldy when I just had it serviced 6 months ago?

Six months is too long for Singapore. Most aircons start showing mould signs at the 4 to 5 month mark in our climate. This is why we recommend quarterly (every 3 months). Not because we want more business, but because that’s what actually works in Singapore’s humidity. The “service every 6 to 12 months” advice you see online is written for temperate climates and doesn’t apply here.