Smelly air conditioner: identifying the odour, tracing its source and dealing with it for good

You switch on the air conditioner and an unpleasant smell fills the room. Sometimes it fades after a few minutes, sometimes it does not. Either way, it is telling you something: a clean, properly installed air conditioner does not smell of anything.

The good news is that each smell has a fairly precise origin. Describing it accurately tells you whether a simple step will do, whether a thorough clean is needed, or whether you should switch the unit off straight away.

Why an air conditioner ends up smelling

In cooling mode, the indoor unit's coil is colder than the room air. Moisture in the air condenses on it, runs into a tray, then drains away through a pipe. That is normal and useful: it is how the unit dries the air.

When the unit stops, the coil stays damp, out of the light, at room temperature. Fine dust and cooking particles that get past the filters settle on it. Together, these conditions encourage mould and a bacterial biofilm, which gradually spreads to the fan, the bladed cylinder that pushes air into the room, and to the condensate tray. On restart, air passes through these deposits and carries their smell with it.

Describing the smell to trace its source

What you smell	Most likely source	What to do
Musty, cellar, damp laundry	Mould on the coil, fan or filters	Clean the filters, then plan a full clean
Sewage, rotten eggs	Condensate drain connected to waste water without a trap, or a dried-out trap	Have the drain connection checked
Sour, vinegar, "dirty socks"	Bacteria settled on the coil, often when heating mode starts	Clean with disinfection
Tobacco, cooking, perfume	Odours trapped in the filters and deposits	Clean the filters and coil
Chemical smell in the first days	New coatings and insulation	Wait a few days, report it if it persists
Burning, hot plastic	Electrical overheating	Switch off at the circuit breaker and call for help

A burning smell is never solved by cleaning. Cut the power and contact our air conditioner repair service.

Conversely, do not rely on your nose to detect a refrigerant leak: in domestic air conditioning, a leak usually goes unnoticed by smell. It is the drop in performance and the readings that reveal it.

The special case of heating mode

Many people first notice the smell in autumn, the first time they switch to heating. That is no coincidence. Deposits built up over the summer on a damp coil are suddenly warmed, and their odorous compounds are released all at once. Defrost cycles, during which the unit briefly reverses its operation, can also bring moisture back onto dirty surfaces.

A faint smell of warm dust that disappears within minutes is nothing to worry about. A sour smell that returns every time the unit starts, on the other hand, means the coil needs a thorough clean before the cold season.

Checks you can do yourself

Without opening the casing or touching the connections, you can already:

• wash the filters in lukewarm water, without harsh detergent, and let them dry completely in the shade before refitting them; during the season, every two to four weeks;
• dry the coil before switching off: let the fan run on its own for about fifteen minutes after cooling, or use the automatic drying function if your model has one;
• check the drainage: with the unit running in cooling mode, water should flow steadily from the end of the drain pipe;
• refill the trap if the drain joins that of a little-used sink or basin: running the tap for a few seconds is sometimes enough to get rid of the sewage smell.

The air conditioner foam cleaner cans sold in supermarkets can freshen the front face of the coil. They reach neither the fan nor the tray, where most of the deposits often sit. Follow the instructions, never spray towards electrical components, and do not pour bleach into the tray: it damages some materials without removing the biofilm.