Cassette dripping onto desks: drain sludge, not a crack

Water dripped from the cassette unit onto the desks below, and a cracked pan would mean lowering the unit from the ceiling. From the floor, a cracked pan and a blocked drain look the same. The question is whether water leaks through the pan or spills over its edge.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Daikin Cassette14 years oldIndustrialChoa Chu Kang, Singapore

Concern: If the drain pan had cracked, the cassette would have to come down from its ceiling mount for a pan replacement.
Found: Drain line fully blocked with sludge causing water to back up into the pan and overflow, pan itself intact
Key check: Flushed drain line and inspected pan surface before condemning the pan
Result: The leak stopped the moment the drain line was flushed. The cassette kept running on its original drain pan, with no ceiling work or unit removal.

What we were told

Water has been dripping from the indoor unit onto the desks below. Desks have had to be moved and a bucket put underneath, and the water keeps coming whenever the unit is running. The unit is old, so full replacement was also on the table.

What we checked

Before assuming the pan had cracked, we checked the drain line. We poured a small amount of water into the pan to see whether it left through the outlet. Nothing came out, which pointed to a blockage. We then went over the pan surface for cracks or corrosion. A cracked pan leaks through its surface, while a blocked drain backs up and overflows the edge.

The drain line was completely blocked with thick sludge. No water flowed out of the drain.
Drain water was backing up inside the pan and spilling over its lowest edge.
After the drain line was flushed clear, water drained freely and the overflow stopped at once.
The pan surface showed no cracks, fractures, or corrosion. The pan body was sound.

What we found

The drain line had slowly blocked with sludge, a mix of algae, bacteria, and dust that builds up over years of use. In a ceiling-mounted cassette, the drain line runs flat before a vertical drop, and sludge gathers along that flat run where water moves slowest. Once it blocked fully, the water backed up in the pan and spilled over the edge onto the desks below. It was going over the pan, not through it. Dripping seen from the floor does not separate the two. Where the water escapes the pan is what settles it.

What fixed it

We flushed the drain line with pressurised water until the sludge cleared and water ran freely, then poured test water into the pan to confirm drainage was restored. No ceiling work was needed, no parts were replaced, and the unit stayed on its mounting. We advised adding a drain flush to every service. On a cassette running long office hours, that is the best way to prevent a repeat leak.

Outcome

The leak stopped the moment the drain line was flushed. The cassette kept running on its original drain pan, with no ceiling work or unit removal.

What this case teaches us

A dripping cassette is usually a blocked drain, not a cracked pan

Pour a little water into the pan and watch the drain outlet. If nothing comes out, the line is blocked and the pan may be fine.
Overflow spills at the pan edge, a cracked pan leaks through its surface. Which one it is decides between a quick flush and a costly teardown.
On a cassette running long office hours, add a drain flush to every service. It is the cheapest way to stop a repeat leak.

Questions this case answers

Cassette aircon dripping onto desks: is the drain pan cracked?

Not always, and the difference decides the whole job. In this Choa Chu Kang office the pan surface was sound, with no cracks, fractures or corrosion anywhere on it. The drain line had packed solid with sludge, so water backed up and spilled over the pan's lowest edge. A cracked pan leaks through its surface. Overflow goes over the rim.

How do I know the drain line is blocked before agreeing to lower the unit?

Pour a little water into the pan and watch the drain outlet. Nothing came out here, which settled it before anyone touched the ceiling mount. Once the line was flushed with pressurised water, the pan drained freely and the overflow stopped at once. That check takes minutes and can spare you a full ceiling removal.

Why does sludge build up in a ceiling cassette drain?

Algae, bacteria and dust collect where water moves slowest. A cassette drain runs flat before its vertical drop, and that flat stretch is exactly where sludge gathers. Fourteen years of office hours gave it time to close the line completely. Adding a flush to every service is the cheapest way to keep it open.

Does an old cassette leaking water mean it should be replaced?

Age on its own proves nothing about a leak. This unit was fourteen years old and full replacement was on the table, yet the fault was a blocked line rather than a worn-out machine. It kept running on its original pan with nothing lifted out of the ceiling. Establish where the water escapes before deciding the unit is finished.

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