Aircon Wall Unit Drip Algae Drain Tray

Case Details

What We Checked

• Drain tray had a thick layer of green algae and biofilm covering the entire tray surface and blocking the drain outlet.
• Condensate was pooling in the tray with no path to the drain line — water was spilling over the front edge.
• After cleaning the tray and clearing the outlet, water drained freely into the drain line.
• Tray surface was inspected after cleaning — no cracks, holes, or corrosion found.

The Diagnosis

Algae and biofilm had built up in the drain tray over several years of use. The growth gradually covered the drain outlet until water could no longer flow out. Condensate backed up in the tray and overflowed from the front of the unit. The tray itself was structurally intact.

What Fixed It

GOOD NEWS — the drain tray was not cracked. The drip was caused by algae blocking the drain outlet. We cleaned the tray, cleared the biofilm from the outlet, and flushed the drain line to confirm flow. No parts were needed.

The drip stopped immediately after the tray was cleaned and the drain outlet cleared. The unit resumed normal operation with its original drain tray. The parquet floor was dried and no lasting damage was found.

Why This Happens

Why drain trays block up — and what causes algae growth.

• The drain tray sits under the evaporator coil and collects condensate. In warm, humid conditions like Singapore, the standing moisture creates an environment where algae and biofilm grow. Over time the growth covers the drain outlet and blocks water from flowing out.
• A choked drain tray overflows from the front of the unit. This looks alarming but does not mean the tray is cracked. Cleaning the tray surface and flushing the drain outlet restores normal flow.
• Regular servicing that includes drain tray cleaning prevents algae from building up to the point where it blocks the outlet. Units in bedrooms are especially prone because the tray stays damp overnight when the unit runs during sleep.