Factory Cassette Dripping Again: Dust Sludge in the Drain

A factory cassette drips again after clearing, in premises nobody can change. Whether the drain is the problem or the supply to it.

By Team Snowflake | Updated 24 Aug 2026

Teaching case based on recurring Snowflake service patterns; identifying details are combined.

How this presents

Dripping from a unit in an industrial setting, with the drain line choked by a thick sludge. The coil above it is heavily loaded. Both have been cleared before, and both have returned to exactly this state.

Setup: Mitsubishi Electric Cassette7-9 yearsIndustrialPioneer
Misread as: A drain that will keep choking whatever is done to it, in premises that cannot be changed.
Check first: The condition of the coil above, as a predictor of the line below it

What else produces this

A drain full of the same material that is sitting on the coil is not two faults. Water condensing on the coil washes deposit off it continuously, and everything that comes off has one place to go. That makes the coil the leading indicator, because it fouls first and it can be seen. A drain that keeps choking on premises like these is reporting the air, and the coil reports it earlier.

Sludge in the drain matching the deposit on the coil identifies one material and one source.
A heavily loaded coil above a choked line shows which of the two arrives first.
Industrial air supplying continuous fine material explains the rate at which both foul.
Both previously cleared and both back in the same state confirms the interval rather than the work.

Why it happens

The coil stays wet the whole time the unit runs, and that water carries away whatever has settled on the fins. In ordinary air the quantity is small and the drain handles it indefinitely. Where the air is heavily loaded, the same process delivers far more material into the line than it can carry. The drain then closes on a timescale the air sets, rather than one the plumbing does.

What resolution involves

Both are cleared together, and the intervals for both are set from the coil's condition rather than from a calendar. The coil is the practical gauge, since it can be seen and it fouls first, so a coil loading quickly is a warning that the drain is filling behind it. Reducing what reaches the unit at source does more for both than any cleaning schedule will.

How to tell it worked

The coil's condition at the next inspection, used to decide whether the drain needs attention. A coil loading fast means the line is filling, whether or not it has started backing up yet.

Before you approve a quote

The coil tells you what the drain is collecting

Water running off the coil carries everything it collected straight into the drain line.
The coil fouls first and can be seen. Use it as the warning that the drain is filling.
Clear both together. Clearing one and leaving the other guarantees an early return visit.

Questions this case answers

Cassette started dripping again soon after it was serviced: why?

Because the water that condenses on the coil runs off it carrying whatever has settled there, and all of that goes into the drain. The two are one system, even though they are usually maintained as separate jobs.

Does a leaking cassette mean the drain pump has failed?

Not necessarily; here the line was choked by sludge washed off the coil above it, not by a failed pump. Checking the coil's condition shows what the drain is collecting and whether the line is filling behind it.

How can a cassette look clean at the grille and still leak?

The grille is not where the problem sits: the coil above it fouls first, and the deposit washes from there into the drain line, which chokes out of sight. Both need clearing together, and the coil is the gauge for the next interval.

What should factory maintenance include beyond normal servicing?

Reducing what reaches the unit in the first place, through better filtration at the return, sealing the space from the dirtier area, or improving extraction there. All of them extend both intervals at the same time.

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