System-3 one zone warm due to indoor-side restriction
Aircon case in Sembawang, Singapore: airflow traced to evaporator coil and blower wheel heavily fouled in affected indoor unit only — airflow restricted to near zero after targeted diagnosis checks.
Case details
What client reported
The customer said the master bedroom had been getting progressively warmer over the past month. The living room and second bedroom remained cold. No error codes showed on any unit. They had read that low gas could affect one zone and wanted it topped up.
What we found
We confirmed the other two zones were cooling correctly before opening the affected indoor unit
- Living room and second bedroom indoor units tested — both producing cold air at normal airflow. This confirmed the outdoor unit and shared refrigerant circuit were functioning correctly.
- Affected bedroom indoor unit checked — blower motor running but airflow at the vents was barely perceptible
- Evaporator coil inspected after removing the front panel — coil face heavily blocked with compressed dust and visible mould across the fin surface
- Blower wheel removed — significant dust accumulation had reduced the effective blade area, causing poor airflow despite the motor running
The affected bedroom's indoor unit coil and blower wheel had built up significant fouling. The other two units had not. The bedroom was used with the door closed most of the time. It had also been serviced less recently than the other units. Reduced air movement combined with infrequent servicing allowed the buildup to reach the point of severe airflow restriction.
What we did
A thorough chemical wash of the affected indoor unit's coil and blower wheel was performed on-site. After cleaning, airflow from the bedroom unit matched the other two zones. Refrigerant pressures were checked as a final step — both within normal range. No gas was lost and no top-up was needed.
The bedroom cooled normally after the cleaning. No parts were needed and the refrigerant system was not touched. The customer was advised to service all three indoor units on the same schedule. This prevents one unit from falling behind the others.
Timeline
Day 1
Customer reported one bedroom not cooling while other two zones fine, outdoor unit running normally
Day 1
Affected indoor unit opened — coil was visibly blocked with dust and mould. Blower wheel had significant buildup that reduced airflow despite the motor running.
Day 1
Other zones confirmed normal, affected indoor unit coil and blower found heavily fouled, chemical wash performed, zone restored
What we learned
Why one room in a system-3 can stop cooling while others are fine.
- In a multi-split system, all indoor units share the same outdoor unit and refrigerant circuit. But each indoor unit has its own coil and blower wheel that can foul independently.
- A refrigerant or outdoor unit fault affects all indoor units at once. The shared circuit cannot selectively deprive one zone. If two zones cool normally, the refrigerant circuit is not the problem.
- Each room's usage pattern is different. A bedroom with the door closed and little ventilation will accumulate coil and blower fouling faster than a regularly aired living room.
- When one zone in a multi-split fails, check that indoor unit's airflow, coil, and blower first. Do this before checking the outdoor unit or refrigerant pressures.
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