5 Reasons Your Aircon Keeps Leaking After Servicing

Flushing the drain line is the standard response to a leaking aircon. Most of the time it works because the most common cause of indoor leaks is a clogged drain. But when the cause is mechanical rather than a blockage, flushing clears the symptom without fixing the problem. The leak stops temporarily because the water has somewhere to go — until the underlying issue fills the tray again.

A recurring leak after servicing does not mean the technician did a bad job. It means the drain flush addressed the effect, not the cause. The five reasons below are the most common culprits when a leak returns within days of a service visit.

During a chemical wash or deep clean, the drain tray is removed for scrubbing. When it goes back in, it needs to sit flush against the base of the fan coil unit. If the tray is even slightly misaligned, water pools at the wrong end and spills over the edge instead of flowing toward the drain outlet.

This is a handling issue, not a part failure. The tray itself is fine — it is just not seated correctly. A quick check after reassembly catches this every time, but under time pressure it is easy to miss. If the leak pattern changed after servicing — dripping from a different spot or from the opposite side of the unit — tray alignment is the first thing to check.

Most aircon drain systems include a U-trap or a check valve that prevents air from being sucked back into the drain line. That trap holds a small amount of water as a seal. When the system is idle for a long time or the drain line is blown out with high-pressure air during servicing, the water in the trap can evaporate or get pushed out.

Without the water seal, air pressure from the fan coil unit pushes back through the drain line and prevents condensate from flowing out. Water backs up in the tray and overflows. The fix is simple — pour a small amount of water into the trap to restore the seal — but it is easy to overlook if the technician flushed the line and moved on.

A standard drain flush clears the section of pipe closest to the fan coil unit. But the drain line runs from the tray through the wall, along a concealed route, and out to the external discharge point. A blockage further downstream — in the wall cavity or at the external outlet — is not reached by a short flush from inside.

This is more common in older HDB flats where the drain line shares a common riser or passes through a long horizontal run. Algae buildup, debris, or sediment can accumulate at bends and junctions that are well past the reach of a simple flush. Clearing these requires access to the downstream section, which may involve work from the outdoor side or access to the riser.

Drain pans develop hairline cracks over time, especially in older units. A cracked pan leaks water before it reaches the drain outlet, so flushing the drain has no effect — the water is escaping from the pan itself, not from a blockage.

Hairline cracks are difficult to spot during a routine service because they are thin and often hidden under residual moisture. The crack only becomes visible when the pan is dry and examined closely. If your unit is older and the leak persists despite a clear drain line, ask the technician to inspect the pan surface with the unit turned off and the pan dried.

Condensate drains rely on gravity. The pipe must slope downward continuously from the drain tray to the discharge point. If any section of the pipe is level or slopes upward — even slightly — water sits in that section and backs up toward the tray.

Gradient issues happen during installation or after renovation work that shifts the pipe route. They also appear in units where the mounting bracket has loosened over time, changing the angle of the indoor unit and the tray. A gradient problem is not something a drain flush can fix. The pipe path needs to be adjusted so water flows in one direction without pooling.

When you book a follow-up, skip the general description and share specifics. Tell them when the leak returned after the last service, where the water appears, and whether the drip location changed. Mention if the unit was idle for any period before the leak started again.

These details help the technician bypass the standard drain flush and look at the mechanical causes listed above. A second visit that repeats the same steps as the first will produce the same result. The goal is to move past the obvious fix and address what is actually causing the water to escape.