🧊 Aircon Maintenance

Musty Smell, Jelly or Mould? 7 Signs Your Aircon Needs Servicing

A musty smell, slimy jelly in the drain, black spots on the vents or water dripping onto the floor all point to the same thing: dirt and mould have built up inside your aircon. Here is what each sign means and what to do about it.

Most of the warning signs below mean that dust, mould or slime has built up inside your aircon, past the filter, where you cannot reach it yourself. City Cooling explains what each one means and when it is time to book a service.

Below: a quick verdict for each sign, the signs that mean you should switch off straight away, and what is behind a musty smell, aircon jelly, mould, dripping, weak airflow, noise and rising bills.

1

Quick verdict: what does each sign mean and what should you do?

Use this table to decide what to do. Cleaning the filter is the only fix most homeowners can safely do themselves. Everything else means the dirt or fault is inside the unit.

Sign	Likely cause	Can you fix it yourself?	What to book
Musty or mouldy smell	Mould on the coil, blower or drain tray	Clean the filter first; if the smell stays, no	Chemical wash at minimum
Aircon jelly (slime)	Biofilm in the drain tray and pipe	No	Service with a drain flush
Black spots on vents	Mould on the louvres and blower	Wipe the vents only	Chemical wash or overhaul
Water dripping	Blocked drain pipe or tray	No	General service or leak check
Weak or warm air	Dirty filter, dirty coil or low gas	Clean the filter first	Service, then a gas check if needed
Rattling or grinding noise	Loose part or debris inside	No	Inspection or repair
Rising electricity bill	Dirty filter and coil	Clean the filter first	General service

Not sure which deep clean you need? See chemical wash or chemical overhaul.

2

Which signs mean switch off and call a technician now?

Some signs are not a servicing question. If you notice any of these, switch the aircon off at the wall isolator or breaker, do not switch it back on, and call a technician:

A burning smell, or a fishy smell like hot plastic. It can point to an electrical fault.
Sparking, crackling or a tripped breaker when the aircon starts.
A hissing sound. This can be a refrigerant leak. See aircon gas leak for what happens next.
Water dripping near a power point or onto electrical sockets.

3

Why does my aircon smell musty?

A musty smell usually means mould is growing on the cooling coil, the blower wheel or the drain tray inside your aircon. These parts stay warm and damp between uses, which is why the smell is often strongest when you first switch the unit on. Cleaning the filter alone rarely removes it.

Where the smell comes from

Air passes over a damp coil and a drain tray full of condensate every time the aircon runs. In Singapore’s humidity, mould grows on damp coils, filters and drain trays, and the blower blows the smell into the room.

What you can do yourself, and when to book

Start by cleaning the filters. If the musty smell comes back within a few days, the source is behind the filter, on the coil, blower or drain tray. A chemical wash is the minimum fix for a musty smell. If the smell returns within weeks of a chemical wash, a full aircon chemical overhaul is likely needed.

4

What is aircon jelly and why does it form?

Aircon jelly is a slimy, jelly-like build-up that forms in the drain tray and drain pipe of an aircon. It is not really jelly: it is a biofilm of bacteria, mould and dust that feeds on the damp inside the unit. As it grows, it blocks the drain, which leads to water dripping and musty smells.

What causes jelly in an aircon

Every aircon produces condensate water while it cools. That water collects in the drain tray and runs out through a small pipe. Dust that gets past the filter settles in the damp tray and pipe, and bacteria and mould grow on it. The longer the drain goes without cleaning, the thicker the jelly becomes.

Can you remove aircon jelly yourself?

We do not recommend it. Pouring bleach, vinegar or other household chemicals into the drain pipe rarely clears an established blockage, and the chemicals can damage the drain tray and pipe. Jelly is flushed out of the drain during a professional service. What you can do is keep the filters clean, which slows the build-up, and keep up regular servicing.

5

Is there mould in my aircon? How to tell

City Cooling technician cleaning dismantled aircon fan coil parts

The clearest signs of mould in an aircon are black or grey spots on the vents, louvres or blower, a musty smell when the unit runs, and a smell that comes back soon after you clean the filters. Mould you can see on the outside usually means there is more on the coil and blower inside.

You can wipe the vents and front panel with a damp cloth. Never spray water or mould cleaner into the unit. Anything inside the aircon, including the coil and the blower wheel, should be left to a technician, who can dismantle and chemically clean those parts.

6

Why is my aircon dripping water?

Water dripping from the indoor unit is usually caused by a blocked drain pipe or drain tray, and aircon jelly is a common culprit. Less often, a dirty coil or low refrigerant causes ice to form and then melt. Left unchecked, the water damages walls, flooring and furniture, so book a technician rather than waiting. Our guide to aircon leaking water covers the causes, repair process and costs in detail.

7

Why is my aircon blowing weak or warm air?

Weak or warm airflow usually starts with a dirty filter, so clean the filters first. If the air is still weak with a clean filter, the coil may be blocked and need a chemical wash, or the unit may be low on refrigerant. If cleaning the filter does not improve cooling within 30 minutes, the issue lies elsewhere. See aircon not cooling: 8 common causes for the full list of causes.

8

Is a noisy aircon a sign it needs servicing?

Sometimes. A rattling sound can mean a loose fan blade or debris inside the unit, which a technician can usually fix during a service. Grinding or loud buzzing from the outdoor unit is more often a repair job. A hissing sound is different and is covered in the switch-off signs above. If your unit shows an error code or a blinking light as well, see aircon error codes by brand.

9

Can a dirty aircon raise your electricity bill?

Yes. Clogged filters and dirty coils restrict airflow, so the compressor has to run longer to reach the same temperature, and that uses more electricity. If your bill has crept up without any change in how you use the aircon, compare a few months of bills and book a general service.

City Cooling's BCA-certified technicians handle aircon servicing in Singapore for HDB flats, condos, landed homes and offices, 7 days a week. If you have spotted any of these signs, book a service or WhatsApp us on 9890 9677. You can see our rates on the price list, and our guide on how often to service your aircon sets out a schedule that helps prevent jelly and mould.

Frequently Asked Questions (FAQs)

Is aircon jelly harmful?

Aircon jelly is a sign that bacteria and mould are growing in the drain tray and pipe. Left alone, it blocks the drain and leads to leaks and musty smells, so have it flushed out during a service rather than waiting for water to drip.

How often should I service my aircon to prevent jelly and mould?

Regular servicing is the best prevention, and how often depends on how many hours a day your aircon runs. Our guide on how often to service your aircon sets out the schedule for Singapore homes.

Will a general service get rid of a musty smell?

Usually not. A musty smell means mould on the coil or drain tray, and a chemical wash is the minimum fix. If the smell returns within weeks of a chemical wash, a full chemical overhaul is likely needed.

Why does my aircon smell when I first turn it on?

The coil and drain pan stay warm and damp between uses, which lets mould and bacteria grow. When you switch the aircon on, the blower pushes that smell into the room. If it happens every time, book a chemical wash.

Can I use an aircon cleaning spray or drain tablet myself?

We do not recommend it. Sprays and household chemicals rarely reach the build-up properly and can damage the drain tray and pipe. Clean the filters yourself and leave the coil, drain tray and pipe to a technician.

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