Aircon Cassette Leak, Blocked Drain

Case Details

What We Checked

• Drain line was completely blocked with thick sludge — no water was flowing out of the drain at all.
• Condensate was backing up inside the pan and overflowing at the lowest edge.
• After flushing the drain line clear, water drained freely and the overflow stopped immediately.
• Drain pan surface was inspected — no cracks, fractures, or corrosion found on the pan body.

The Diagnosis

The drain line had gradually blocked with biological sludge — a mixture of algae, bacteria, and dust that forms naturally in condensate lines over years of operation. In a cassette unit mounted in a ceiling void, the drain line runs horizontally or at a slight incline before reaching a vertical drop. Sludge accumulates at the lowest point of the horizontal run where water velocity is slowest. Once the blockage became complete, condensate had nowhere to go. It backed up inside the drain pan, filled to the brim, and overflowed at the lowest edge of the pan housing — dripping onto the desk equipment below. The drain pan itself was structurally intact with no cracks, fractures, or corrosion. The water was going over the pan, not through it. The previous contractor saw the dripping and concluded the pan had cracked, but never checked whether the drain line was flowing.

What Fixed It

We flushed the drain line with pressurised water until the sludge cleared and water flowed freely from the outlet. Then we poured test water into the pan to confirm drainage was restored and checked the pan surface one more time to give the client confidence it was structurally sound. No ceiling work was needed, no parts were replaced, and the unit did not need to be lowered from its mounting. We recommended adding drain flushing to every scheduled service visit going forward — for a commercial cassette unit running long hours, this is the single most effective step to prevent a repeat. The client saved the cost of an unnecessary pan replacement and ceiling repair.

The leak stopped immediately after the drain line was flushed. The cassette unit continued operating with its original drain pan. No ceiling work or unit removal was needed.

Why This Happens

Cassette leaks — cracked pan vs blocked drain.

• A cracked drain pan leaks through the pan surface even when the drain line is clear. A blocked drain causes water to back up and overflow the pan edges. Both produce dripping from the unit — but the fix for one is a simple flush, while the other requires lowering the unit from the ceiling. Getting the diagnosis wrong is expensive.
• Before condemning the drain pan, the drain line should be flushed and the pan surface inspected for cracks. If flushing restores drainage and no cracks are found, the pan was never the problem. Ask your technician to demonstrate that the drain line flows before accepting a pan replacement quote.
• Cassette units in commercial spaces develop drain blockages faster than residential units because they run longer hours and process more condensate. The horizontal drain runs common in ceiling installations are especially prone to sludge buildup at low points where water moves slowly.
• A fourteen-year-old unit with a sludge blockage is not at end of life — it is overdue for drain maintenance. Regular drain flushing as part of scheduled servicing prevents blockages from escalating into leaks and avoids unnecessary pan replacement quotes.