Why your AC smells in the monsoon, and how to stop it coming back

The sour, damp smell when your AC starts up is mould. The indoor coil, the blower wheel behind it and the drain tray underneath have been wet for months, and in Nellore's monsoon — October to December, with humidity high from July onwards — they never get a chance to dry. Each time the unit starts, the first minute of air carries the smell of whatever has grown there into the room.

It is not dangerous in the way a gas or burning smell is, but it is a sign the unit is dirty in a place you cannot see, and a dirty coil also cools worse and uses more power. The fix is to dry it out daily and clean it properly once.

Where the smell comes from

A split AC cools the room by passing air over a coil that is colder than the dew point, so water condenses on it constantly and runs down into a tray and out through the drain pipe. In dry weather, when the AC switches off, the coil dries within an hour. In the monsoon the air in the room is so humid that the coil stays wet all night, the tray holds a film of water, and the dust that passed through the filters sits on that wet surface as food.

Three things make it worse in Nellore:

• Long humid season. The coil is damp from July to December, not just for a few weeks.
• Hard water. In localities on borewell supply the condensate leaves scale in the tray and the drain, which traps slime and slows the drain until water sits.
• Dust. Near main roads and open ground, fine dust gets past the filters and coats the coil, giving growth something to hold on to.

What you can do tonight

1. Clean the filters. Slide them out, rinse, dry in shade, refit. Dirty filters let more dust reach the coil and reduce the airflow that would help it dry.
2. Run Fan mode before switching off. For the last ten to fifteen minutes before you turn the AC off, switch to Fan only. The blower pushes room air over the coil and dries it. Many units have a self-clean, X-dry or auto-dry setting that does exactly this when you press Off; use it if you have it.
3. Use Dry mode on muggy days. When the room is sticky at 29°C rather than hot at 34°C, Dry mode pulls moisture out without overcooling and the room feels comfortable sooner.
4. Check the drain outside. If water drips from the indoor unit or the pipe outside is barely trickling while the AC runs, the drain is partly blocked and the tray is holding water. That is a service job, not something to poke at with a wire.
5. Do not spray perfume or "AC freshener" into the grille. It masks the smell for a day and adds another film to the coil.

What you should not do

Do not open the indoor unit beyond the filters. The coil fins bend easily, the blower wheel is awkward to reach, and the electrical board sits right next to the wet parts. Do not pour anything into the drain tray from inside, and do not run the AC with the front panel open to "dry it faster". Switch off at the MCB before touching anything near the unit with wet hands.

The post-monsoon service

The smell is the clearest argument for servicing twice a year in Nellore rather than once. The pre-summer service gets the unit ready for heat; the post-monsoon service, in late October or November, clears out what three humid months have left behind. For ₹399 the coil is washed through with water under pressure from the correct side, the blower wheel is cleaned, the tray is scrubbed and the drain line flushed, and the filters are done properly. If the smell is gone after that service and back within a few weeks, the drain is not clearing and the tray is refilling; say so and have the drain slope checked.

Smells that are not mould

A few smells mean something else and should not be ignored:

• Burning or hot plastic. Switch off at the MCB. A motor, a connection or the board is overheating. Book a repair and mention the smell.
• Sharp, chemical, or sweet. Possibly a refrigerant leak at the indoor coil. Switch off, ventilate the room, and book; a diagnosis visit is ₹350 and a refill after a leak repair is ₹700 on a split.
• Sewage or drain smell. The drain pipe may be terminating somewhere it should not, or a dry trap is letting air back up the line.

Keeping the smell away next year

Service after the monsoon, clean filters monthly, run Fan mode for ten minutes before switching off through the humid months, and make sure the drain outside runs freely. An AC that is dried every night has very little for mould to live on, and the first breath of air in October smells of nothing at all.