Why does my aircon smell? 5 causes and how to fix each

Why does my aircon smell?

Aircon smells usually come from mould and biofilm growing on the wet evaporator coil, blower fan and drain pan, or from room odours the coil has absorbed. A thorough clean of those parts removes the smell; air fresheners only mask it. A burning or electrical smell is different and needs a technician straight away.

Key takeaways

• Mould grows where it is wet: the evaporator coil condenses water every time the aircon runs in Singapore's humidity.
• The room matters too: new wooden furniture, drying laundry, pets, scented oils and body powder all feed the smell.
• The filter is only the first layer: washing the air filter clears dust but not the coil, blower fan or drain pan behind it.
• The fix depends on where the slime is: light build-up needs servicing, a slimy fan and drain pan need a chemical wash, and a chemical overhaul is the last resort.

In the video above, Uncle Eric goes through five everyday reasons an aircon starts to smell stale or mouldy, and shows how we fixed a unit less than a year old whose smell came from a new wooden bed frame in the room.

How can I tell where the smell is coming from?

1. Note when it smells. If it only smells for the first few minutes after you switch on, water sat in the coil and drain pan while the unit was off: early build-up. If it smells all the time, the build-up is heavier.
2. Check the air filter. If it is grey and furry, wash it and run the aircon for a day. If the smell is gone, the filter was the problem. If it comes back within a day or two, the source is behind the filter.
3. Look at the blower fan. Shine a torch through the air outlet. Black spots or slime on the blades mean the fan is a source and has to come out to be washed.
4. Look for water. Dripping, or a sour, stale-water smell, points to a drain pan or drain pipe that is not draining properly.
5. Think about the room. New furniture, laundry drying indoors, pets, oil diffusers or heavy use of powder mean the coil is soaking up odours.
6. Burning, hot plastic or electrical smell? Switch off at the isolator and do not run the unit until a technician has checked it.

If steps 3 or 4 point to slime or standing water, a filter wash will not fix it. Book your next service so the coil, blower fan and drain pan can be cleaned properly.

Where does the smell actually come from?

The indoor unit is a cold, wet place. As humid air passes over the evaporator coil, water condenses on the fins and runs down into the drain pan. Any dust that slips past the filter sticks to those wet fins. Over months it turns into a jelly-like slime, a biofilm that mould and bacteria feed on. Every time the fan spins, your room air passes straight over it.

The blower fan, the long barrel-shaped drum behind the louvres, collects the same slime between its blades. The drain pan, if it does not empty fully, holds water that turns sour, and the same blockage often ends in a water leak.

Why does a fairly new aircon smell?

A new aircon can still smell, because the coil picks up whatever is in the room. In the video, Uncle Eric lists five common reasons:

1. New wooden furniture. The glue and laminates in a new bed frame or wardrobe give off fumes that the coil soaks up and blows back out.
2. Wet towels or laundry drying in the room. The damp smell goes into the aircon and adds moisture to an already wet coil.
3. Pets. Fur and the animals themselves leave odour and hair on the coil and fan, especially when servicing is skipped.
4. Essential oils, scented candles and aerosol air fresheners. The oils and fine droplets settle inside the unit.
5. Body powder, make-up and fragrance. Used heavily in a bedroom, they coat and clog the evaporator coil, and the smell then comes off the blower fan.

None of these happens overnight. The smell usually takes months to build up, and a unit that is not serviced regularly gets there faster. In the bed-frame case from the video, the unit was under a year old; the smell was strong every time it was switched on, and a PowerJet chemical overhaul cleared it.

What kind of smell is it?

Can I get rid of the smell myself?

You can slow it down. Wash the air filter every two to four weeks if the unit runs every night. Avoid drying laundry in the aircon room, and air out a room with new furniture before running the aircon for long hours. Some newer units have a self-dry or clean function that runs the fan after you switch off; use it if yours has one.

What you cannot do yourself is clean the coil and blower fan properly. Uncle Eric is blunt about the DIY foam sprays sold in hardware shops: in his experience they do not clear a dirty coil.

How do we fix a smelly aircon?

We open the unit first and look at the coil, fan and drain pan, because the right fix depends on how deep the build-up goes.

• Light build-up: premium servicing with PowerJet. The evaporator coil gets a PowerJet deep clean, the blower fan and drain pan come out and are jet-washed, and the drain is flushed. See how a PowerJet wash works.
• Slime on the fan and in the drain pan: a chemical wash, with the unit still on the wall. The blower fan and drain pan come out, are PowerJet-washed and go back in as-new condition, while coil chemical loosens the dirt and grime on the coil and a PowerJet deep clean flushes it out. For a very dirty coil, a heavy-duty coil cleaner from S$30 is advised on site.
• An extremely soiled unit, with slime inside and behind the coil: a chemical overhaul, as the last resort, where the unit is taken down and stripped, as in the bed-frame case.

Our guide to chemical wash vs chemical overhaul explains the difference in full. You can also watch: smelly Mitsubishi Electric Starmex fixed with PowerJet Wash. Every service comes with a 90-day warranty.

What it costs

To stop the smell returning, servicing on schedule matters most; see how often to service your aircon in Singapore.

When to call a technician

• The smell returns within a few days of washing the filter.
• You can see black spots on the blower fan, or specks blow out of the vents.
• The smell comes with water dripping or weaker cooling.
• Anyone at home is sensitive to dust or mould and the smell will not go away.
• You notice a burning or electrical smell. Switch off at the isolator first.

Book online in under a minute with instant confirmation, or ask Luna, our AI assistant, on WhatsApp any time, day or night.