Two Sides Strong, Two Sides Weak: Blower Caked on One Side

Two sides of a cassette blow strongly and two barely at all. What an asymmetric symptom rules out before anything is taken down.

By Team Snowflake | Updated 24 Aug 2026

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

How this presents

A ceiling unit pushing well from two of its four outlets while the opposite pair barely moves the air. The room warms on the weak side and holds temperature on the strong one. Filter and outlets look ordinary from below.

Setup: Hitachi Cassette7-9 yearsIndustrialSeletar
Misread as: Something structurally wrong with the ceiling unit that would need it taken down.
Check first: Comparing all four outlets against each other before anything is opened

What else produces this

Anything shared across the whole unit fails evenly. Refrigerant, fan speed, the compressor, the filter: none of them can favour two outlets over two others. So an asymmetry rules out the entire shared side of the system before anything is opened, which is why the pattern is worth establishing carefully at the start. What remains is something that occupies part of the unit and not the rest.

Two outlets moving air normally prove the fan, the charge and the cooling side are all doing their work.
Two outlets weak while sharing that same fan places the restriction inside the unit rather than upstream.
Vanes moving through their full travel on all four sides removes the mechanical explanation.
A blower wheel caked heavily along one arc and clear elsewhere matches the outlets that are failing.

Why it happens

A cassette draws air in through the middle and throws it outward, and where the intake pulls unevenly the deposit builds unevenly with it. One arc of the wheel loads while the rest stays reasonably clear. The loaded arc throws less, and because each part of the wheel feeds the outlets nearest it, the shortfall appears on those sides only. The fault is ordinary fouling arranged in an unusual pattern.

What resolution involves

The wheel is cleaned across its full circumference rather than only where the deposit is visible, since a partial clean rebuilds the same imbalance within months. Balance returns alongside airflow. The transferable part is the reasoning rather than the repair: an uneven symptom from a single unit is a gift, because it eliminates everything the unit does as a whole.

How to tell it worked

All four outlets tested and matched against each other, not just the two that were weak. Comparing one side against another is what proves the imbalance is gone rather than merely reduced.

Before you approve a quote

An uneven symptom from one unit points inside it

Refrigerant, fan and compressor are shared. None of them can fail on two outlets and not the others.
Establish the pattern before opening anything. Which sides are weak is the most useful fact available.
Clean the whole wheel, not the visibly dirty arc. A partial clean recreates the same imbalance.

Questions this case answers

Two vanes on the cassette blow strong and two blow weak: why?

Because deposit has built up along one arc of the blower wheel, and each part of the wheel feeds the outlets nearest it, so the loaded arc throws less air on those sides only. A fault in the fan, the charge or the coil would affect all four outlets equally.

Can dust alone cause uneven airflow in a cassette?

It can, and the vanes are worth checking first because it takes seconds. A vane stuck part-closed weakens its own outlet, but it leaves the wheel behind it clean, which is how the two are told apart.

How do I know it is not the dampers or the motor?

Watch all four vanes through a full sweep, which takes seconds: every one moved through its full travel here, which clears the dampers. The motor is shared by all four outlets, so the two strong sides clear it; that leaves something inside the unit, and the wheel was caked along one arc.

Will a chemical clean fix lopsided cassette airflow?

Only if the whole wheel is cleaned. Where a technician works on the visibly dirty arc alone, the remaining deposit keeps the imbalance and the same complaint returns within months.

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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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