Aircon System-3 One Zone Not Cooling

Case Details

What We Checked

• 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 Diagnosis

The affected bedroom's indoor unit had accumulated heavy fouling on both the evaporator coil and blower wheel. The other two indoor units in the system had not reached this level. The bedroom was used primarily with the door closed, which limited natural air circulation and increased moisture retention around the coil. It had also been serviced less recently than the other two units — the client had staggered services rather than cleaning all three together. The combination of reduced air movement, higher local humidity, and a longer gap since the last cleaning allowed dust and mould to compress on the coil face and blower blades until airflow was severely restricted despite the motor running normally.

What Fixed It

A thorough chemical wash of the affected indoor unit's evaporator coil and blower wheel was performed on-site. Cleaning solution was applied to dissolve the compressed dust and mould buildup, then flushed out completely. After cleaning, airflow from the bedroom unit was tested and matched the output of the other two zones. Refrigerant pressures were checked as a final verification step — both high-side and low-side readings fell within normal range for the ambient conditions, confirming that the shared refrigerant circuit had never been the issue. 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 client was advised to service all three indoor units on the same schedule. This prevents one unit from falling behind the others.

Why This Happens

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.