Aircon Shop Unit Dripping, Drain Algae

Case Details

What We Checked

• Indoor unit drain pan was full but not damaged.
• No visible crack or deformation on the unit housing.
• Drain pipe running along the warm ceiling had no flow at the outlet end.
• Disconnecting the pipe revealed thick algae growth blocking the interior.

The Diagnosis

The drain pipe ran horizontally through a warm, humid space above the ceiling tiles before reaching the external outlet. This environment — warm air, constant moisture from condensation, and minimal airflow — created ideal conditions for algae growth inside the pipe. Algae colonies formed along the pipe walls and thickened over months until the internal diameter was effectively sealed. With no drainage path available, condensate water backed up through the pipe, filled the drain pan beyond capacity, and overflowed from the indoor unit onto the shop floor below. The indoor unit itself was structurally intact — no cracks, no pan damage. The drip was entirely caused by the downstream blockage preventing normal drainage.

What Fixed It

We explained that the indoor unit was structurally fine — no crack, no pan damage, no internal leak. The dripping was caused entirely by algae blocking the drain pipe along the warm ceiling section. We flushed the pipe with pressurized water until flow ran clear at the outlet end, then placed an algae inhibitor tablet at the drain pan inlet to slow regrowth. We recommended adding a drain flush to every routine service visit going forward, given the length and warmth of this particular pipe run. We also suggested the shop owner consider an algae-resistant drain pipe sleeve for the ceiling section if the problem recurred despite regular flushing.

After the drain pipe was flushed and treated, water flowed freely to the outlet. The dripping stopped and the unit ran dry through the rest of the day. No parts were replaced.

Why This Happens

Why warm drain pipe runs breed algae blockages.

• Drain pipes running along warm ceilings or through enclosed spaces create the perfect environment for algae — warmth, constant moisture, and still air. In commercial units where pipe runs are often longer and more horizontal than residential setups, the risk of algae blockage is significantly higher.
• When algae thickens enough to seal the pipe, water backs up into the drain pan and overflows from the indoor unit. The drip pattern looks identical to an internal crack or pan failure, which is why some technicians jump to a housing replacement diagnosis without checking the pipe run first. Ask whether the drain pipe outlet was tested for flow before any indoor unit repair is quoted.
• Flushing the drain line with pressurized water and applying an algae inhibitor tablet at the drain pan inlet clears the blockage and slows regrowth. This is a maintenance task, not a repair — no parts are replaced and the indoor unit is untouched.
• For commercial units with long or horizontal drain runs, scheduling a drain flush during every routine service visit is the most effective prevention. The warmer and more enclosed the pipe route, the more frequently the flush is needed — quarterly is a reasonable starting point for shophouse installations.