Why Is My Aircon Leaking Water Inside?

Condensate that drains from the evaporator coil carries fine dust, airborne biological matter, and dissolved minerals. Over months — and in Singapore's climate, it can be a matter of weeks in units that run continuously — this material settles in the drain tray and pipe and forms a slick biofilm. As the biofilm thickens, the drain cross-section narrows. Water begins to queue in the tray rather than flowing freely. Once the level reaches the tray lip, it spills over and drips from the indoor unit casing — typically at the front edge or along the connecting pipe entry point.

The misdiagnosis risk is treating this as a refrigerant or cooling system fault because the dripping appears while the unit is running. Cooling may still feel acceptable in early-stage blockages — the drain issue and the cooling performance are independent. Technicians who skip a drain-flow test and proceed to check refrigerant pressures or coil condition waste time and add cost. The tell is the drip pattern: steady, consistent, tied to runtime, and not linked to any change in airflow or temperature output.

A cracked or misaligned drain pan can mimic a blocked drain. If the pan is cracked from age, water escapes before reaching the pipe. If the pan shifted out of position after servicing or a bump, water pools on one side and spills. The drip in both cases comes from a fixed spot — but it starts immediately rather than building over weeks. If a leak appeared right after a service visit, a displaced pan is worth checking first.

• –Steady drip pattern during runtime.
• –Gurgling water sound near the indoor unit.
• –Cooling can still feel acceptable at first.
• –Drip always from the same fixed point on the casing.

A blocked drain causes a steady leak during the run cycle. The tray fills, water backs up, and it drips from the same spot each time. That differs from freeze-thaw, which gives sudden heavy bursts after shutdown, and from slope or routing faults, where the drip point can shift along the trunking. If cooling is still acceptable and the leak is constant, start with the drain path. We clear and flush the drainage path, correct slope or insulation issues where needed, and retest under cooling load to confirm leaking stops. Gas work will not solve a pure drain obstruction pattern. Confirm drain behavior before approving unrelated scope.

The evaporator coil is designed to operate between roughly 5°C and 15°C — cold enough to condense moisture from the room air, but above freezing. When airflow through the coil drops sharply — from a blocked filter, a dust-laden coil surface, or a slow blower motor — or when refrigerant charge is insufficient, the coil surface drops below zero. Moisture freezes on contact with the fins instead of draining away. Ice builds up across the coil, blocking airflow further and accelerating the cooling loss. When the system shuts down or the low-pressure safety trips, the ice block melts rapidly and discharges a volume of water that the drain tray cannot handle. The result is water pouring from the indoor unit in a sudden burst.

This pattern is almost always misidentified as a drain blockage because the water appears from the same place. The distinguishing feature is timing: a pure drain blockage produces a steady drip during operation, while freeze-thaw produces bursts — typically after shutdown or after a period when airflow was noticeably weaker. Clearing the drain without identifying what drove the coil below freezing point means the freeze cycle repeats, and each subsequent event pushes more water than the last as the underlying restriction worsens.

Low refrigerant from a slow pipe leak produces a similar freeze pattern through a different path: the coil gets too cold because there is not enough refrigerant to absorb heat properly. Ice forms, melts when the system cycles off, and the water volume overwhelms the drain tray. The key distinction is that cleaning the coil does not stop it — if the coil is clean but ice keeps forming, a pressure check for low refrigerant is the correct next step.

• –Leak appears in bursts, not a constant flow.
• –Cooling weakens before leakage worsens.
• –Icing signs may appear near the indoor area.

The freeze-thaw pattern produces water in sudden, heavy bursts — often after the unit shuts down or after a period of noticeably weaker cooling — rather than the steady drip that characterizes a drain blockage. Unlike the slope or routing fault where moisture appears in varying locations along the trunking, a freeze-thaw event typically floods the drain tray at once from a melting coil. If you see a large volume of water suddenly appearing, cooling that weakened before the leak worsened, or any signs of icing on the pipe, freeze-up is the active suspect — not a blocked drain. We remove the root restriction causing freeze-up, restore airflow or refrigerant balance as needed, and retest for stable operation without icing. Treating every leak as a simple drain issue can miss the freeze trigger and lead to repeat leakage.

The condensate drain system depends on gravity to move water from the indoor unit tray to the external outlet. The tray must slope toward the drain pipe entry, and the pipe itself must run consistently downward without dips or horizontal runs where water can pool and stagnate. When installation was done with insufficient attention to slope — common in tight ceiling-mounted or concealed unit installations — water sits in the tray and eventually finds an alternative exit path: along the bottom edge of the trunking, through a gap in the pipe insulation, or along the wall lining.

The drip point in these cases shifts depending on which path has accumulated enough water to overflow first. That moving drip location is the key diagnostic indicator — a blocked drain drips from a fixed point, while a slope or routing fault produces moisture at different positions along the trunking run. Replacing the drain pipe or adding a pump without correcting the slope addresses the symptom but not the geometry. In HDB units where the indoor unit is installed at the far end of a long trunking run, this problem is more common than most technicians acknowledge.

On very humid days, moisture from room air can also condense on the cold outer surface of the indoor unit casing — this is not a fault. The bigger the temperature gap between the room and the unit surface, the more condensation forms. If this happens daily or excessively, poor insulation around the piping connection may be letting cold air escape, which a technician can wrap and seal.

• –Drip location shifts along trunking or wall path.
• –Leak timing changes with weather or runtime.
• –Moisture appears even when cooling output seems normal.

The defining indicator of a slope or routing fault is that the drip point moves — moisture appears at different positions along the trunking or wall lining over time, rather than consistently at one fixed spot as in a blocked drain. Unlike the freeze-thaw path where the failure is thermal and appears in bursts, this path is geometric: water follows the lowest available route through poorly sloped piping. If the drip location varies between rain events, runtime periods, or operating modes, trace the geometry of the drain installation before flushing the line. We clear and flush the drainage path, correct slope or insulation issues where needed, and retest under cooling load to confirm leaking stops. Replacing major components without confirming water-path behavior can add cost without solving recurrence.