Waterfront office smells musty: drain tray film, not gas

A Marina East office unit smelled musty when it started after idle weekends. The space still cooled, so the diagnosis had to separate moisture buildup inside the fan coil from unrelated gas or parts work.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Mitsubishi Electric Wall-mounted4 years oldOfficeMarina East, Singapore

Concern: The tenant worried the smell meant hidden gas leakage or a failing indoor unit.
Found: Moisture film in the drain tray and blower area after idle periods
Key check: Checked tray, blower, and startup smell before recommending gas or parts
Result: The startup smell cleared after the wet areas were cleaned. The case stayed small because the diagnosis followed the timing of the smell instead of jumping to gas or replacement. If the smell returns, the useful clue is whether it appears only after idle days or during every cooling cycle.

What we were told

The office unit smelled musty for the first few minutes after startup, especially after the space had sat unused over a weekend. Cooling was acceptable once the unit ran. The tenant asked whether gas could be leaking or whether the unit needed replacement.

What we checked

Because the unit still cooled and the smell peaked at startup, we started with moisture and airflow rather than the refrigerant side. We checked the filter, blower face, drain tray, and whether water was sitting where it should leave the unit. Waterfront humidity and idle-room use made moisture film more plausible than a major part failure.

The smell was strongest during the first startup minutes.
The drain tray had a visible film and stale moisture marks.
The blower edge had light buildup that could hold odour.
Cooling response did not point to an immediate gas problem.

What we found

The smell came from moisture film around the tray and blower area. When the room sat unused, moisture stayed in the indoor unit long enough to create a musty smell at startup. Once the fan ran for a while, the smell diluted, which matched a local indoor source rather than a constant gas or electrical issue. The unit needed targeted cleaning of the wet areas that hold odour.

What fixed it

We cleaned the tray and blower area, checked that water left the drain path, and advised running fan mode briefly after heavy cooling when the room would be left unused. No gas top-up or part replacement was recommended because the cooling side was not the main clue. The tenant was told to track whether the smell returns on startup only or stays throughout operation; that distinction helps decide the next check.

Outcome

The startup smell cleared after the wet areas were cleaned. The case stayed small because the diagnosis followed the timing of the smell instead of jumping to gas or replacement. If the smell returns, the useful clue is whether it appears only after idle days or during every cooling cycle.

What this case teaches us

Startup smell usually needs moisture checks first

A musty smell on startup is usually about trapped moisture, tray film, blower buildup, or room humidity. It is not a gas diagnosis by default.
Tell the technician whether the room sits unused. Idle rooms can hold moisture around the tray and blower even when cooling still works.
Cleaning should target the source of the smell. Fragrance or repeated filter washing will not solve tray and blower film.

Questions this case answers

Why does my office aircon smell musty when it starts on Monday?

Idle time is the reason. Over a weekend with the unit off, moisture stayed inside the fan coil long enough to turn stale. The drain tray in this Marina East office had a visible film and stale moisture marks, and the blower edge carried light buildup that holds odour. Once the fan had run a while, the smell diluted.

Can a musty aircon smell mean refrigerant is leaking?

Refrigerant was not the pointer here. Cooling response was acceptable throughout and the smell was strongest in the first few minutes rather than constant, which fits a local damp source inside the indoor unit. A smell that fades as the fan keeps running behaves nothing like a gas or electrical problem, so the wet areas were cleaned instead.

Will air freshener or washing the filter get rid of the smell?

Neither reaches the source. Fragrance covers the odour while the film stays, and repeated filter washing leaves the tray and blower untouched. Cleaning has to target where the moisture actually sits. After the tray and blower area here were cleaned and the drain path confirmed, the startup smell cleared.

What helps a room that sits unused for days between cooling?

Run fan mode briefly after heavy cooling before the space is left empty, so the wet surfaces inside dry rather than sitting damp. Waterfront humidity makes that more worthwhile. Also worth tracking: whether the smell shows only on startup or lingers right through operation, because that difference decides the next check.

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Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

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