You turn on your aircond expecting a blast of cool, fresh air, and instead you are hit with a musty, damp smell that reminds you of old socks or a wet towel left in the laundry basket. It is one of the most common complaints we hear from homeowners across Petaling Jaya and the Klang Valley, and it is more than just unpleasant — it can indicate a genuine health concern.
That musty smell means something is growing inside your aircond. In Malaysia’s hot and humid tropical climate, air conditioners are essentially the perfect breeding ground for mould, bacteria, and fungi. Understanding what causes the smell and how to eliminate it permanently is crucial for your comfort, health, and the longevity of your aircond system.
What Causes the Musty Smell?
The musty odour from your aircond is almost always caused by biological growth — specifically mould, mildew, bacteria, or a combination of all three. Here is where they grow and why.
The Evaporator Coil
The evaporator coil is the primary cooling component inside your aircond’s indoor unit. When warm, humid air from your room passes over the cold coil, moisture condenses on the coil surface — exactly like water droplets forming on a cold glass of teh tarik. This constant moisture, combined with the dust and organic particles that settle on the coil, creates an ideal environment for mould and bacteria to thrive.
In Malaysia, where ambient humidity levels regularly exceed 70 to 90 percent, the amount of condensation on the evaporator coil is substantial. A single residential aircond unit can produce 1 to 3 litres of condensate water per hour during peak operation. All that moisture feeds the microbial growth that causes the smell.
The Drain Pan
The drain pan sits beneath the evaporator coil and collects all the condensate water before channelling it to the drain line. When the pan is not cleaned regularly, a layer of slimy biofilm (a mixture of bacteria, algae, and organic matter) develops on the pan surface. This biofilm is a major source of musty odour, and it also restricts water flow, increasing the risk of overflow and water leakage.
The Drain Line
The condensate drain line carries water from the drain pan to the outside of your home. Over time, algae and slime grow inside the drain line, gradually narrowing the passage. A partially blocked drain line causes water to pool in the drain pan, which amplifies the musty smell. A fully blocked line causes water to overflow from the indoor unit entirely.
The Blower Fan
The blower fan (also called the turbine fan or barrel fan) spins at high speed to blow cooled air into your room. Dust and moisture accumulate on the fan blades over time, and mould colonies form in the damp crevices. Every time the fan spins, it disperses mould spores and musty-smelling particles into your room along with the cooled air.
Is the Musty Smell Harmful to Health?
Yes, it can be. The musty smell is a symptom of mould and bacterial contamination, and breathing in these microorganisms poses real health risks, especially with prolonged exposure:
Allergic reactions. Mould spores are a common allergen. People who are sensitive may experience sneezing, runny nose, itchy eyes, and skin irritation when the contaminated aircond is running.
Respiratory issues. Prolonged exposure to mould and bacteria from a dirty aircond can aggravate asthma, cause persistent coughing, and contribute to upper respiratory infections. Children and elderly family members are particularly vulnerable.
Sick Building Syndrome. In office environments, contaminated aircond systems are one of the leading causes of Sick Building Syndrome, where occupants experience headaches, fatigue, and difficulty concentrating that improve when they leave the building.
A study by the Malaysian Medical Association noted that indoor air quality in air-conditioned spaces is a significant concern in tropical countries like Malaysia, where the warm, humid climate promotes faster microbial growth inside HVAC systems than in temperate countries.
How to Fix the Musty Smell
Temporary Fix: Run Fan Mode for 30 Minutes After Cooling
One reason the smell is strongest when you first turn on the aircond is that moisture sits on the coil and drain pan when the system is off. Running the fan mode (without cooling) for 15 to 30 minutes after you turn off the cooling function helps dry out the coil surface, reducing the moisture available for mould growth.
Some modern airconds from Daikin and Panasonic have an “auto-dry” or “mould prevention” feature that does this automatically. If your unit has this feature, enable it. If not, make it a habit to switch to fan mode before turning the unit off completely at night.
Effectiveness: This reduces the rate of mould growth but does not eliminate existing contamination. It is a preventive measure, not a cure.
Temporary Fix: Clean the Filters and Spray Disinfectant
Cleaning the air filters removes surface dust and some of the smell. You can also spray a light mist of anti-bacterial aircond spray (available at hardware stores and Mr. DIY for RM 15 to RM 25) onto the coil through the front panel opening after removing the filters.
Effectiveness: This provides temporary relief for a few days to a few weeks, but it only treats the surface. The real contamination is deep inside the coil, drain pan, and blower fan, where sprays cannot reach adequately.
Permanent Fix: Professional Chemical Wash
The only way to permanently eliminate the musty smell is a thorough chemical wash performed by a qualified technician. Here is what the process involves:
1. Preparation. The technician covers the surrounding wall and furniture with plastic sheeting to protect against chemical splash and dirty water.
2. Disassembly. The front panel, filters, and air direction louvres are removed for individual cleaning.
3. Chemical application. A specialised alkaline cleaning solution is sprayed onto the evaporator coil. This chemical breaks down the biofilm, mould, grease, and dirt that have accumulated on the coil fins. The solution is left to soak for several minutes to penetrate deep into the coil.
4. High-pressure rinse. The technician uses a pressurised water sprayer to flush the dissolved contaminants out of the coil. The dirty water (often black or dark brown) collects in a drainage bag attached below the unit. First-time customers are often shocked at how much grime comes out of a seemingly normal-looking aircond.
5. Drain pan and line cleaning. The drain pan is scrubbed clean and the drain line is flushed to ensure clear drainage.
6. Blower fan cleaning. The blower fan blades are cleaned with chemical solution and rinsed. In cases of severe contamination, the technician may need to remove the blower fan entirely for soaking.
7. Reassembly and testing. All components are reinstalled, and the system is tested for cooling performance, airflow, and drainage.
After a proper chemical wash, the musty smell is completely gone. The air from your aircond will smell clean and neutral, the way it should.
How to Prevent the Smell from Coming Back
Once your aircond has been professionally cleaned, these habits will help keep the musty smell from returning:
Clean filters every two weeks. This prevents dust from reaching the evaporator coil and feeding mould growth.
Use the dry mode or fan mode. Run the fan for 15 to 30 minutes after turning off cooling to dry the coil surface. This simple habit significantly slows mould regrowth.
Schedule chemical washes every 6 months. In Malaysia’s humid climate, even a well-maintained aircond will develop some biofilm within 6 months. Regular chemical washes prevent it from reaching the point where you can smell it.
Ensure proper drainage. Periodically check that your condensate drain is flowing freely. If you notice any water pooling or slow drainage, have it cleared before mould develops.
Maintain indoor humidity. If your home is particularly humid (common in ground-floor apartments and houses near water bodies in the Klang Valley), consider using a standalone dehumidifier to reduce overall moisture levels. Lower humidity means less condensation and slower mould growth.
Keep the area around the indoor unit clean. Dust and airborne particles in the room eventually end up inside your aircond. Regular household cleaning, vacuuming, and minimising dust sources help reduce the amount of dirt that enters the system.
When the Smell Indicates a Bigger Problem
In most cases, a musty smell is resolved by a chemical wash. However, if the smell persists or returns within a few weeks after cleaning, there may be a deeper issue:
Mould inside the wall cavity. If moisture has been leaking from the aircond into the wall over time, mould may have established itself inside the wall behind or around the unit. This requires wall inspection and remediation beyond aircond servicing.
Damaged drain pan. Some older units develop cracks in the drain pan, allowing water to seep into areas that are not accessible during normal cleaning. A cracked pan needs to be replaced.
Severely corroded evaporator coil. If the coil has extensive corrosion, the rough surface provides more attachment points for mould and bacteria, making it difficult to clean completely. In severe cases, coil replacement may be necessary.
Get Professional Help in PJ and Klang Valley
If your aircond smells musty, do not just live with it. The smell is telling you that mould and bacteria are contaminating the air you breathe every night. A professional chemical wash from PJ Kool Aircond Service will eliminate the problem and restore clean, fresh air to your home.
We serve all areas across Petaling Jaya, Subang Jaya, Shah Alam, Damansara, and the wider Klang Valley. Our chemical wash service includes gas top-up at no additional charge, and we provide same-day or next-day appointments for most areas. Contact us today to schedule your service.
Eliminate musty smells for good: Our aircond chemical wash service removes mould, bacteria, and grime from every internal surface — same-day appointments available.
