Aircon Cassette Uneven Cooling, Stuck Vane

Case Details

What We Checked

• Three of four louver vanes moved freely — normal operation.
• One vane on the warm-corner side was seized near-closed — the motor was running but the linkage had locked up.
• Coil section behind the stuck vane had visible scale and dust not seen on the other three sections.
• Refrigerant pressures checked — both sides within normal range, charge correct.

The Diagnosis

The vane pivot had seized from dust and moisture accumulating on the linkage mechanism over two years without servicing. Cassette units draw room air upward through the centre and push cooled air outward through four directional vanes. Each vane swings on a small pivot driven by a stepper motor and plastic linkage. When dust and moisture coat the pivot, friction increases until the motor can no longer move it. With one vane stuck near-closed, airflow through that quarter dropped to almost nothing. The reduced airflow also meant less evaporation on that section of the coil, so condensation lingered and attracted more dust. This created a feedback loop of fouling that made the warm spot progressively worse over the two months the client noticed it.

What Fixed It

We cleaned the seized pivot and linkage mechanism with a degreasing solvent, then applied a light lubricant rated for the operating temperature. All four vanes were cycled multiple times to confirm smooth, full-range movement. The fouled coil section was cleaned to match the condition of the other three sections. We ran the unit for fifteen minutes and checked airflow at each of the four outlets with a hand-held anemometer — all four quadrants were balanced within normal range. No parts were replaced and no refrigerant work was done. We recommended moving to a six-month service interval to prevent the pivot from seizing again in the ceiling-void environment.

Even cooling was restored on the same visit. No parts were needed. The client was advised to service the cassette every six months. Cassette units sit in sealed ceiling voids and foul faster than wall-mounted units.

Why This Happens

Cassette uneven cooling — vane fault vs. system fault.

• A cassette unit has four directional vanes, each controlling airflow to one quarter of the room. If one vane is stuck, that quarter loses airflow while the other three continue cooling normally — creating a pattern that looks like the system is struggling but is actually a single mechanical fault.
• A refrigerant leak or compressor weakness affects the whole unit equally — all four quadrants weaken together. When only one corner is warm, the vanes and that section of coil are the first things to check, not the gas charge.
• Checking each vane takes a few minutes with the cassette panel open. A stuck vane can be seen and felt by hand. Ask your technician to cycle all four vanes before assuming it is a system-level fault — this simple check can save you the cost of unnecessary gas work.
• A stuck vane creates a feedback loop: less airflow over that coil section means moisture lingers, dust accumulates, and fouling builds up. Over time this makes the warm spot worse even though the root cause is just a seized pivot.