Weak airflow after servicing: one fan coil needed deeper work

Weak airflow and a stale startup smell returned in one bedroom soon after routine servicing. The other rooms still cooled normally. That difference mattered: another whole-system package would repeat work where it was not needed, while another surface clean could miss the restriction inside the affected fan coil.

By Team Snowflake | Reviewed 25 Jul 2026

Case summary

Mitsubishi Electric Multi-split10 years oldHDBTampines, Singapore

Concern: The household could approve chemical work in every room when the restriction belonged to one fan coil
Found: Deep blower and coil buildup affecting one fan coil
Key check: Compare the affected blower and coil with normally cooling rooms before choosing the cleaning depth
Result: After the one-unit overhaul, airflow was even and the stale startup smell was gone. The other two fan coils remained outside the deeper cleaning scope.

What we were told

One bedroom took longer to cool and smelled stale when first switched on. Its filter looked clean after recent routine servicing. The living-room and second-bedroom units moved air normally and reached a comfortable condition, which kept the complaint centred on one indoor unit rather than the shared outdoor system.

What we checked

We started with a room-to-room comparison before considering another service package. All three units ran at the same settings. The two working rooms gave us a baseline for airflow, fan sound, and cooling response. We then opened the affected unit and checked its filter, coil, blower, drain tray, and fan response in that order.

The living-room and second-bedroom units produced even airflow and cooled normally on the same outdoor system.
The affected filter and exposed coil face looked clean, but compacted material remained deeper across the coil and blower.
The fan started and changed speed normally, which pointed away from a motor fault as the main cause of weak airflow.
Moist buildup around the inner coil and blower matched the stale startup smell, while the drain still cleared.

What we found

We found compacted buildup behind the visible coil face and across the blower. The earlier surface clean had left this deeper material in place, so the fan could spin normally without moving enough air through the unit. Normal cooling in the other rooms also ruled against a shared outdoor fault. The restriction belonged to one fan coil, not the complete system.

What fixed it

We recommended dismantling and chemically overhauling only the affected fan coil. The internal condition was recorded before the coil and blower were cleaned, reassembled, and checked for airflow, drainage, sound, and cooling. The living-room and second-bedroom units stayed outside the overhaul scope because their checks did not show the same restriction. Cleaning depth followed the evidence in each room.

Outcome

After the one-unit overhaul, airflow was even and the stale startup smell was gone. The other two fan coils remained outside the deeper cleaning scope.

What this case teaches us

A clean filter does not settle a returning airflow problem

Compare the affected room with normally cooling rooms on the same system before treating weak airflow as a shared outdoor fault.
Ask to see the blower, inner coil face, drain tray, and fan operation. A clean filter does not show those areas.
The report should show what was opened, what remained after surface cleaning, and why only one fan coil needed dismantling.

Questions this case answers

Why is my airflow still weak after a recent aircon service?

Surface cleaning reaches the filter and the exposed coil face, not material packed deeper across the coil and blower. That is what happened here: the fan spun at normal speed and still could not move enough air through the unit, because the restriction sat behind everything the earlier clean had touched.

Does one weak room mean the whole system needs chemical work?

Not on its own. The other two rooms here ran on the same outdoor system and produced even airflow and normal cooling. That comparison kept the problem inside one indoor unit and kept the other two out of the overhaul scope. Without it, a whole-system package looks reasonable by default.

Is a stale smell when the aircon starts a sign of deep buildup?

It can be, but check where the moisture is sitting. Here the smell matched moist buildup around the inner coil and blower, while the drain still cleared normally. A tray or drain problem produces the same complaint with a much lighter fix, so the two need separating before a wash is agreed.

What should the report show if only one unit gets dismantled?

It should show what was opened, what was still there after the earlier surface clean, and why the other units stayed out of scope. The internal condition here was recorded before cleaning, then airflow, drainage, sound, and cooling were checked after that one fan coil went back together.

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