Musty smell on every start-up: biofilm across the coil

A sour, damp smell arrived with the first minute of cooling and faded after that. That pattern points at something growing on the wet parts inside, and it does need chemical cleaning. It does not automatically mean the unit comes off the wall.

By Team Snowflake | Reviewed 25 Jul 2026

Case summary

Mitsubishi Electric Wall-mounted5 years oldHDBTampines, Singapore

Concern: Having the unit taken off the wall and removed from the flat over a smell, with no inspection to confirm the inside was fouled.
Found: Biofilm growth across the lower evaporator fins and the drain pan, all of it reachable with a chemical wash in place
Key check: Lifted the front panel and ran a finger along the lower fin edge, which came away carrying a grey slime rather than dry dust
Result: The smell was gone from the next start-up onward. Chemical wash was the correct call here, and the overhaul was not. We suggested running fan-only for a short spell before switch-off so the fins dry.

What we were told

The bedroom smelled sour for the first stretch of cooling every night, then settled. Filters had been rinsed at home more than once with no change. A contractor smelled it, said the inside was contaminated beyond normal cleaning, and quoted for the unit to be taken down and soaked.

What we checked

Smell diagnosis starts with when the smell appears, so we ran the unit from cold and stood at the vent. It came through sour in the opening minutes, then thinned out as the coil chilled. That timing points to organic growth wetted by condensate, warming up before the coil catches up. We opened the panel to see how deep it had gone.

Grey slime coated the lower third of the fin face and the pan below it. The upper fins were clean and dry to the touch.
Airflow was full strength and supply air was properly cold, so heat exchange was not blocked.
The drain pan held standing water with a film on the surface. The pipe itself ran clear when flushed.

What we found

Condensate keeps the lower fins and the pan wet for hours every night. In a bedroom that runs on a timer, the unit stops while those surfaces are still damp, and nothing dries them before the next cycle. Mould and bacteria settle into that film and build a soft layer. It smells before it ever affects cooling, which is why the odour arrived long before any drop in performance.

What fixed it

We recommended a chemical wash with the unit left on the wall. Foaming solution was worked through the fins and left to lift the film, then rinsed until the runoff came clear. The pan and pipe were treated in the same pass. Dismounting would have added cost and a reinstallation risk for buildup that was sitting on the surface, well within reach of a wash.

Outcome

The smell was gone from the next start-up onward. Chemical wash was the correct call here, and the overhaul was not. We suggested running fan-only for a short spell before switch-off so the fins dry.

What this case teaches us

A smell earns a wash, not always a strip-down

Smell that fades once cooling settles means growth on wet surfaces, which chemical solution dissolves in place.
Smell that stays strong the whole time is a different problem, often drainage or something in the room.
Dismounting is for buildup that a foaming wash cannot reach, judged by looking at the fins, not by the odour.

Questions this case answers

Why does the aircon smell sour for the first few minutes and then stop?

The fade is the clue. A smell that arrives cold and thins out as the coil chills is growth sitting on wet surfaces, not deep contamination. Condensate leaves the lower fins and pan damp overnight, and a film settles there. In this Tampines bedroom grey slime coated the bottom of the fin face while everything above it stayed clean.

Does a musty smell mean the unit has to be taken down and soaked?

Not on the strength of the smell alone. Here an overhaul was quoted off the odour, with the unit to be dismounted and soaked off site. Opening the panel showed the film sitting on the surface of the lower fins and the pan, well within reach of a foaming wash carried out in place.

Will rinsing the filter myself get rid of the smell?

It did not here. The filters had been rinsed at home more than once with no change, because the growth was past them on the fin face and in the pan. Filters catch dust travelling in. They do not touch the damp surfaces behind, which is where the film builds and where the smell comes from.

What can I do to stop the smell coming back?

Let the fins dry before the unit stops. We suggested running fan-only for a short spell before switch-off, so the condensate on the lower fins and pan is not left sitting damp all night. A bedroom unit on a timer cuts out while those surfaces are still wet, which is what let the film establish.

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