Water marks on ceiling: drain pump switch stuck by sludge

Water marks on a ceiling below a ducted unit point to a drain pump that has stopped clearing water. The motor and the float switch that starts it are separate parts. A stuck float is a far smaller fix than a new pump.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Daikin Ducted5 years oldLandedTanglin, Singapore

Concern: The homeowner feared the whole drain pump had to be replaced and the ceiling opened up to reach it.
Found: drain pump water-level switch stuck in the down position due to sludge buildup. The pump could not activate, causing water to overflow the drain pan
Key check: Tested the drain pump operation and inspected the water-level switch mechanism before replacing the entire pump assembly
Result: The overflow stopped and the pump now starts on its own during normal use. The original motor stayed in place and no extra ceiling work was needed.

What we were told

Water marks were spreading on the living-room ceiling below the aircon. The worry was a failed drain pump needing full replacement, and a ceiling panel opened up to reach it. Nothing had been checked at the unit itself.

What we checked

Overflow from a ducted unit can start at the drain pan, the drain line, or the pump. We tested each stage in turn. The motor and its float switch were checked separately, since a pump that runs on the test button can still fail to start on its own.

The pump motor ran normally on the test button. The motor, impeller, and wiring were all working.
The float switch did not move freely through its full range. Buildup around the pivot and guide rod was holding it back.
Sludge coated the float inside the reservoir, a sticky layer of dirt and mineral residue.
The drain pan and drain line were clear, so nothing else in the path was blocking the water.
Water sat near the top of the reservoir, but the float had not risen enough to start the pump. That confirmed the stuck float as the cause.

What we found

Over five years, drain water carrying dust, dirt, and mineral residue left a layer of sludge inside the pump reservoir. It built up around the float switch and slowly limited how far the float could rise. A drain pump float works like a toilet cistern float: it rises with the water and starts the motor at a set height. With its travel cut to less than half, the float could no longer reach that height, even with the reservoir nearly full. The motor was healthy on the test button but never got the signal to start during normal use. Water kept filling the reservoir and spilled onto the ceiling panel, seeping through the joints over days.

What fixed it

We flushed the reservoir with a dilute descaler to dissolve the sludge, then cleaned the float and guide rod so they moved freely again. After reassembly, we poured measured amounts of water in. The float rose correctly and the pump started at the right level. We then ran the aircon at full speed to push peak drain flow. The pump cycled on its own and the drain pan stayed dry. No new pump, no new reservoir, no ceiling work beyond the existing access panel.

Outcome

The overflow stopped and the pump now starts on its own during normal use. The original motor stayed in place and no extra ceiling work was needed.

What this case teaches us

A pump that runs by hand may still fail on its own

A drain pump has two parts that can fail: the motor and the float switch that tells it to start. Check them separately.
The test button only proves the motor runs. It does not prove the pump starts by itself during normal use.
Sludge can jam the float so it never reaches the trigger height. Cleaning it often beats replacing the whole pump.

Questions this case answers

Water marks on the ceiling under a ducted unit: does the pump need replacing?

Not necessarily, because a drain pump has two parts that fail separately. In this Tanglin home the motor, impeller and wiring all worked, while the float switch that tells the motor to start was jammed by sludge. Water sat near the top of the reservoir and the float never rose far enough to trigger it.

The pump runs on the test button, so why is water still overflowing?

The test button only proves the motor turns. It bypasses the float switch entirely, so it says nothing about whether the pump starts by itself during normal use. Buildup around the pivot and guide rod had cut the float's travel to less than half, so the start signal never arrived.

What is sludge doing inside a drain pump reservoir?

Drain water carries dust, dirt and mineral residue, and over five years that settled as a sticky layer inside the reservoir. It gathered around the float, which works like a toilet cistern float and must rise to a set height to start the motor. Restricted travel meant a nearly full reservoir still read as empty.

Can this be fixed without opening the ceiling?

It was here, through the existing access panel. The reservoir was flushed with a dilute descaler, the float and guide rod cleaned so they moved freely, then measured water poured in to check the pump started at the right level. Running the aircon at full speed showed it cycling on its own with the pan dry.

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