Cassette leaking after heavy rain: float switch missed load

A ceiling cassette dripped water, but only during heavy rain, and 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 | Updated 28 Jul 2026

Case summary

LG Cassette4 years oldCondoDowntown Core, Singapore

Concern: A cracked drain pan or an internal leak was the read, and expensive repairs the expected outcome.
Found: Condensate pump float switch trips reliably under ordinary rain but not fast enough at peak heavy-rain load, letting water back up and overflow only in those conditions
Key check: Checked the drain during operation, comparing an ordinary rainy day against a simulated heavy-rain load, and found the float switch trips too late once volume passes its set trigger point
Result: We recalibrated the float switch to trigger earlier under heavy load. 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 heavy-rain day, when the fault was most likely to show.

The drain line was clear. We ran water through it and found no blockage.
The condensate pump and its float switch both worked, but the switch was slow to trip under a heavy load.
On an ordinary rainy day, the pump kept pace and the tray never came close to overflowing.
Under a simulated heavy-rain load, water arrived faster than the float switch was set to trigger, and the tray backed up before the pump caught up.

What we found

The pump and float switch were both healthy under normal conditions. The switch triggers the pump once water in the tray reaches a set level, and during ordinary rain it kept pace. During sustained heavy rain, the unit pulled far more moisture from the air and the tray filled faster than usual. The incoming water outpaced what the switch was calibrated to catch, so the tray rose past the trigger point before the pump responded and spilled at the pan edge. On lighter rain days, the switch never faced that load, so nothing leaked.

What fixed it

We told the owner the drain line and pan were fine. The fault was the float switch calibration, not the drain path. We recalibrated the switch to trigger earlier, well ahead of the tray's overflow point, and confirmed it caught the rise during a simulated heavy-rain load. The drain line and pan needed no work.

Outcome

We recalibrated the float switch to trigger earlier under heavy load. 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 something that struggles under peak load, not a blockage.
The fault here was the float switch trigger point, set for ordinary rain rather than the heaviest days. Under peak load it tripped late and let the tray overflow.
Recalibrating the switch fixed it. The pan and drain line were never the problem, so no major parts were replaced.

Questions this case answers

Ceiling cassette drips only during heavy rain: is the drain clogged?

A blockage leaks whenever the unit runs, not only in a storm, so the rain-only pattern argues against it. We ran water through this drain line and found it clear. What we were chasing was something that struggles under peak load, which is a different kind of problem from something that is blocked.

What is a float switch and how can it cause a leak?

It is the switch that starts the condensate pump once water in the tray reaches a set level. Set to trigger too late, it copes with ordinary conditions but not with a heavy load that fills the tray faster. Water then passes the trigger point before the pump responds and spills at the pan edge.

Does a leaking cassette mean the drain pan has cracked?

Not here. The pan and the drain line were both sound and needed no work at all. Recalibrating the float switch to trigger earlier, well ahead of the overflow point, was the whole repair. No major parts were replaced, which is worth confirming before anyone quotes for a pan or a unit.

Can a leak that only happens in a storm be reproduced during a visit?

Yes, by simulating the load instead of waiting for weather. We compared the drain under ordinary rainy conditions against a simulated heavy-rain load. On the ordinary day the pump kept pace and the tray never came close to overflowing. Under the heavier load, water arrived faster than the switch was set to catch.

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