Cassette Dripping After Cleaning: Debris in the Drain Line

An office cassette begins dripping days after a filter clean. Whether the drain blocked on its own matters more than clearing 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 ceiling unit within days of a filter and coil clean, on a drain that had never given trouble. The clean itself was thorough. Nothing was reported as restricted while it was done.

Setup: Panasonic Cassette7-9 yearsOfficeDowntown Core
Misread as: A drain that has blocked on its own, and that will keep doing so whatever anybody does about it.
Check first: Whether the drain was flushed before the coil work or after it

What else produces this

Something that behaved for years and blocked days after a clean has been given a load it was not handling before. Deposit lifted off a coil has one obvious route out, and that is the tray beneath it. Where the line was already carrying a normal share, a single visit's worth of loosened material is enough to close it. The order of work matters here rather than the quality of it.

A drain with no history of trouble blocking days after a clean points straight back at the work.
A thorough clean explains where a sudden quantity of loose material came from.
Deposit found in the line matching what was lifted off the coil connects the two directly.
Nothing reported as restricted during the work confirms the line was perfectly clear beforehand.

Why it happens

Coil cleaning loosens deposit and washes it downward, and the tray and drain are directly below. In normal running the line receives a trickle of material and clears it. After a clean it receives the accumulation of months in a few minutes. A line with any existing narrowing closes at that point, and it does so a few days later once the loosened material has settled.

What resolution involves

The line is flushed as the last step of the clean rather than the first, so whatever came off the coil is carried out rather than left to settle. Sequence is the entire lesson: clean the coil, then flush the drain. Doing it the other way round leaves a clean line underneath a coil that is about to shed everything into it.

How to tell it worked

Water run through the line after the coil work is finished, watching for it to run away freely at the far end. Flushing before the clean tests a line that is about to receive its real load.

Before you approve a quote

Flush the drain after the coil, never before

Everything lifted off a coil goes somewhere, and the tray beneath it is the obvious route.
A single clean can put more into a drain than a year of normal running does.
Sequence is the fix: coil first, drain second. The reverse leaves a clean line about to be loaded.

Questions this case answers

Cassette aircon dripping after a cleaning visit: what causes it?

Deposit lifted off the coil during the clean washes down into the tray and the drain below it. A line handling a trickle in normal running is suddenly given months of accumulation in a few minutes, and it can close under that.

Ceiling cassette dripping water: is the coil cracked?

Not on this evidence: the drip began days after a coil clean, and the deposit lifted off the coil had settled in the drain line below it; flushing the drain after the coil work is what keeps the line clear.

Why does the dripping only start in the afternoon?

Because that is when the unit works hardest and produces the most condensate, so a line narrowed by loosened material is pushed past what it can carry. The material settles as it dries, which is why the blockage shows up a few days after the clean.

Do I need a new drain pump if water is coming from the panel edge?

No. It means the drain was flushed at the wrong point in the sequence, which is a matter of order rather than of thoroughness. A more careful clean done in the same order produces the same result.

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Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

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