Cassette leaking after heavy rain: drain outlet froze briefly
A ceiling cassette dripped water, but only during heavy rain. The first guess was a clogged drain. A blocked drain leaks whenever the unit runs, so the rain-only pattern did not fit. The timing pointed somewhere else.
By Team Snowflake | Reviewed 14 Mar 2026
Case summary
LG Cassette4 years oldCondoDowntown Core, Singapore
- Concern
- The owner worried the unit had a cracked drain pan or an internal leak that would need expensive repairs.
- Found
- Drain outlet freezing momentarily when cold air stream hits it during peak drain water load
- Key check
- Checked drain during operation and observed frost forming at the outlet during heavy humidity periods
- Result
- We repositioned the drain outlet away from the direct cold air stream. Heavy rain returned two weeks later, and the unit stayed dry. It has held dry through the rainy periods since, with no further drainage problems and no major parts replaced.
What we were told
Water dripped from the corner of the ceiling unit during last week's heavy rain. It stopped when the rain let up. The owner asked whether the drain was clogged, and whether the leak would return during the next storm.
What we checked
A leak that tracks the rain usually means a drain problem. But this one came and went, which a simple blockage does not do. A blockage leaks steadily once water cannot pass. We checked the drain path while the unit ran on a humid day, when the fault was most likely to show.
The drain line was clear. We ran water through it and found no blockage.
The drain outlet sat directly below the cassette, in the path of the cold air it blows down.
On the humid diagnosis day, frost formed at the outlet while the unit ran.
The frost appeared exactly when the indoor coil was producing the most drain water.
What we found
On very humid rainy days, the unit pulled more moisture from the air and made more drain water. Because the outlet sat in the cold air path, frost formed there briefly and slowed the drain. That short backup pushed water up to the pan edge, where it spilled over and dripped. On dry days the unit made less water, the outlet did not frost, and nothing leaked. That is why the leak only followed the rain.
What fixed it
We told the owner the drain line and pan were fine. The fault was the outlet position. Moving it out of the cold air path lets the drain water leave through a warmer route, so it cannot frost and back up. The repositioning is straightforward and avoids replacing any parts.
Outcome
We repositioned the drain outlet away from the direct cold air stream. Heavy rain returned two weeks later, and the unit stayed dry. It has held dry through the rainy periods since, with no further drainage problems and no major parts replaced.
What this case teaches us
When a leak follows the rain, watch the timing
- A clogged drain leaks every time the unit runs. A leak that only shows up during heavy rain points to humidity, not a blockage.
- The fault here was the drain outlet sitting in the cold air stream. On humid days it frosted over and slowed the drain just enough to overflow.
- Repositioning the outlet fixed it. The pan and drain line were never the problem, so no major parts were replaced.
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