Aircon One Room Not Cold, Not Low Gas

Case Details

What We Checked

• Living room and second bedroom indoor units were cooling normally — good airflow, cold coil surface, and room temperature dropping as expected.
• Master bedroom unit had noticeably weaker airflow and the air coming out was barely cool rather than cold.
• Opening the master bedroom unit revealed significant buildup on the evaporator coil — the lower rows were visibly coated with a dust-and-moisture layer restricting heat exchange.
• Blower wheel in the same unit also had moderate buildup, further reducing air volume across the already-restricted coil.
• No signs of refrigerant issues system-wide — outdoor unit operating normally, other indoor units performing well.

The Diagnosis

The master bedroom indoor unit had heavy dust and moisture buildup on its evaporator coil, mostly on the lower rows where condensation collects. This coating blocked heat exchange. The refrigerant flowing through the coil could not cool the air properly. The blower wheel in the same unit also had moderate buildup. It was pushing less air across the already-clogged coil. Together, these two problems — restricted coil and reduced airflow — left the room much colder than it should have been. Because this system shares one outdoor unit, the refrigerant supply itself was fine. The other rooms proved it — they were cooling normally. A system-wide gas top-up would have been wasted money. The real problem was isolated to one indoor unit.

What Fixed It

No system-wide gas work was needed. The refrigerant level was fine — the other rooms proved it by cooling normally with temperature differentials of 8–10°C across their coils. The problem was only in the master bedroom indoor unit. The coil was clogged with a dust-and-moisture layer on the lower rows, and the blower wheel had moderate buildup between its fins reducing air volume. We recommended a chemical wash to clear both surfaces thoroughly — a standard general service would not dislodge the packed-in buildup. After the wash, we retested all rooms to confirm the master bedroom was back in line with the others. We also measured airflow output at the vents post-wash to establish a baseline for future comparison. We explained that this room might need servicing more frequently than the others, since it runs longer hours and accumulates buildup faster.

After the room-specific indoor-side issue was corrected, the warm room cooled normally again. No system-wide work was needed.

Why This Happens

How to tell a room-specific fault from a system-wide gas problem.

• Low refrigerant in a shared multi-split system reduces cooling across all rooms in a roughly proportional way — every indoor coil receives less refrigerant, so every room gets warmer. If one room is clearly worse while the others are still cold, the gas level is almost certainly not the issue. The outdoor unit is delivering refrigerant evenly through the distribution manifold.
• When only one room is affected, the fault is almost always in that room's indoor unit — restricted airflow from blower buildup, reduced heat exchange from coil buildup, or a combination of both. These are unit-specific conditions that do not affect the other indoor units on the same circuit, because each indoor unit has its own coil and blower assembly.
• Comparing room-by-room cooling behaviour before approving any system-wide work is the single most important diagnostic step. Measuring the temperature differential at each indoor coil takes minutes and immediately reveals which unit is underperforming. This prevents paying for a gas top-up that would not fix a room-specific problem.
• Different rooms in the same system can accumulate buildup at different rates depending on usage hours, room dust levels, pet hair, and whether windows are opened frequently. The master bedroom often runs the longest hours — sometimes twelve or more hours daily — and builds up faster than rooms used only a few hours in the evening.