When your AC starts blowing air that smells musty, stale, or like burning, your first instinct is to ignore it and hope it goes away. It rarely does. Bad AC odours usually have a specific cause — and most can be fixed quickly once identified.
Musty or Mouldy Smell (Most Common)
This is by far the most common AC odour complaint in Chennai, especially during and after the monsoon season. It's caused by mould and mildew growing inside the indoor unit — on the evaporator coils, inside the drain tray, or in the drain pipe.
Why it happens: The inside of an AC unit is damp and dark — perfect for mould growth. When air passes over mouldy coils, it picks up the musty odour and spreads it into your room.
Fix: A thorough coil cleaning with an antifungal coil cleaner spray, drain pipe flushing, and drain tray cleaning. After servicing, the smell disappears within a day. Running the fan-only mode for 30 minutes after each use helps dry the coils and prevents mould recurrence.
Burning or Electrical Smell
A burning smell is a serious warning sign. It usually means an electrical component — the motor, capacitor, or wiring — is overheating or burning out.
What to do immediately: Switch off the AC from the breaker (not just the remote) and do not use it until a technician has inspected it. Running an AC with a burning electrical issue can damage the compressor or — in rare cases — cause a fire.
Fix: The technician identifies the overheating component. Common culprits are a failing capacitor (inexpensive to replace) or a motor with worn bearings. Wiring issues are less common but need urgent attention.
Dirty Sock or Feet Smell
This specific odour — sometimes called "dirty sock syndrome" — is caused by bacteria growing on the evaporator coils. Unlike mould (which smells musty), bacteria produce a more pungent, foot-like odour. It's more common in units that are switched off for months and then restarted.
Fix: Antibacterial coil cleaning spray, followed by a thorough service. Running the AC in fan-only mode for 20–30 minutes before switching it off each time helps prevent bacterial growth by drying the coils.
Rotten or Dead Animal Smell
In Chennai, small lizards, insects, and occasionally rodents enter AC units and get trapped — especially in outdoor units left unused for months. A dead animal causes a very strong, unmistakable odour.
Fix: The outdoor unit cover is removed, the dead pest found and removed, and the affected area sanitised. Fitting a mesh over the outdoor unit vents prevents recurrence.
Chemical or Refrigerant Smell
A sweet, chemical smell — sometimes described as ether or nail polish remover — may indicate a refrigerant leak. Refrigerant itself is not harmful in small amounts, but prolonged exposure is not advisable, and the leak needs fixing immediately.
Fix: Switch off the AC and call a technician to locate and seal the leak before recharging the gas.
Cigarette or Smoke Smell
If anyone smokes in the room, the AC filter and coils absorb the smell over time and recirculate it. A thorough filter and coil cleaning is the fix — but if smoking continues indoors, the smell will return.
Prevention: How to Keep Your AC Fresh
- Clean the filter every 3–4 weeks
- Run fan-only mode for 20 minutes before switching off each day
- Service the AC every 3–4 months including coil and drain cleaning
- Keep the area around the outdoor unit clear of leaves and debris
If your AC is producing a bad smell, RK Aircon provides deep cleaning and odour treatment services across Chennai. Most smell issues are resolved in a single visit.