Landed unit drips and smells musty after rain: biofilm in the drain tray

A Seletar landed unit started dripping after several wet evenings, and the owner noticed a faint musty smell near the indoor unit. The room still cooled, so the question was whether biofilm building up inside the drain tray, not a cracked tray or a failed indoor unit, explained both signs.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Daikin Wall-mounted6 years oldLandedSeletar, Singapore

Concern: Owner thought the tray had cracked because water appeared only after wet weather.
Found: Algae and biofilm grown inside the drain tray outlet from trapped moisture, causing both the musty smell and the drip
Key check: Opened the cover and checked the tray outlet and the smell source before quoting tray replacement
Result: The dripping and the musty smell both stopped during the test run and needed no parts. The owner now has a simple rule: report whether the leak or the smell follows rain, long runtime, or both.

What we were told

The living-room unit cooled normally, but water appeared along the lower casing after rainy evenings, and a faint musty smell lingered near the unit on humid days. The owner had wiped the filter and could not see a crack. The drip stopped for a while, then returned the next wet week.

What we checked

Because cooling was still normal, we did not start with gas or major indoor parts. We checked the water path and the source of the smell together: tray condition, tray outlet, drain hose fall, and whether algae or biofilm had built up inside the tray. Seletar's humid, green-fringe air can let moisture sit in a tray long enough for biofilm to form on its own.

The drain tray was intact and did not show a crack.
A layer of dark, slimy biofilm had formed inside the tray around the outlet, giving off a faint musty smell.
Water backed up when the unit ran longer after rain.
The drain hose still had fall, so the blockage and the smell both traced back to the tray itself, not the hose or an outdoor source.

What we found

Ordinary moisture, dust, and organic residue had sat inside the drain tray long enough to grow algae and biofilm around the outlet, not leaf matter carried in from outside. Seletar's humid, green-fringe air kept the tray damp between cycles, letting the biofilm thicken and give off the musty smell. The unit drained fine during short runs, but after wet evenings the moisture load rose and the narrowed outlet overflowed. The tray looked failed from outside because water reached the casing edge, but it was not broken.

What fixed it

We cleared the biofilm from the tray outlet, flushed the drain path, and checked that water left steadily and the smell was gone before closing the unit. We also advised the owner to watch the pattern after heavy rain, not only after long cooling sessions. No tray replacement was quoted because both the drip and the smell stopped once the tray was cleaned out. The next maintenance should include a drain-tray check, not just filter washing.

Outcome

The dripping and the musty smell both stopped during the test run and needed no parts. The owner now has a simple rule: report whether the leak or the smell follows rain, long runtime, or both.

What this case teaches us

A drip with a musty smell points to the tray, not a crack

A leak that appears after wet weather, especially alongside a musty smell, is not automatically a cracked tray. Trapped moisture and organic residue can grow biofilm inside the tray on their own.
Open the cover and check the tray outlet before approving bigger work. A clean front filter does not prove the drain path or tray interior is clear.
In humid, green-fringe homes, ask whether a drip and a smell showed up together. That pairing often points to biofilm inside the tray, not a crack or a failed part.

Questions this case answers

Aircon dripping and smelling musty at the same time: are the two connected?

In this Seletar landed home they shared one cause. A layer of dark, slimy biofilm had formed inside the drain tray around the outlet, narrowing it and giving off the smell at the same time. Water backed up after wet evenings and reached the casing edge, so a single clean-out addressed both signs.

Does water appearing only after rainy weather mean the tray has cracked?

The tray here was intact with no crack visible anywhere. Humid green-fringe air kept it damp between cycles, which let the biofilm thicken around the outlet, and the higher moisture load after wet evenings was simply more than the narrowed outlet could pass. It drained fine on short runs, which is why the fault came and went.

Is a clean filter enough to rule out a drain problem?

No. The owner had wiped the filter and could see no crack, yet the buildup causing both the drip and the smell sat inside the tray behind the cover. Filter washing never reaches it. Opening the cover and checking the tray outlet is what settled this one before any replacement was discussed.

What is worth reporting when an aircon leaks now and then?

Whether it follows rain, long runtime, or both. That pairing narrows things quickly, and combined with a smell it points at the tray rather than the hose or an outdoor source. The drain hose in this home still had proper fall, so the trail led back to the tray itself.

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