Persistent Musty Smell: Mould Deep Inside The Cooling Coil

• •Concern: Worry was that the smell meant something was rotting inside the unit or that the refrigerant was leaking.
• •Key check: Removed filter and blower housing to inspect the coil face. Found visible mould growth across the fin surface

We removed the filter (visibly clean from the recent service) and opened the blower housing for a direct line of sight to the evaporator coil face. In a humid ground-floor shophouse where two rounds of filter cleaning hadn't resolved the smell, the coil was the most likely source. We also checked the drain tray underneath. Stagnant water with biofilm can produce a similar musty odour and is easy to overlook when only the filter is being cleaned.

• •Filter was clean. Recently washed by the previous contractor.
• •Evaporator coil fins had visible grey-green mould growth across the upper third of the surface.
• •Mould colony was established deep between the fins where airflow couldn't dry it out.
• •Drain tray also had a thin biofilm layer contributing to the smell.

Mould had colonised the evaporator coil fins deep between the tightly-spaced aluminium surfaces. In normal operation, the evaporator coil runs cold and draws moisture from the room air, that moisture condenses on the fins and drains into the pan below. But in a ground-floor shophouse with the front door opening frequently, ambient humidity stays high enough that the coil surface never fully dries between operating cycles. This persistent moisture gave mould spores a foothold between the fins where airflow is minimal. Once established, the colony produced the musty smell that the room air carried on every cycle. General servicing only cleaned the filter, which sits in front of the coil. It never contacts the fin surface where the mould was growing.

We recommended a chemical servicing rather than another general service, explaining that the mould colony between the fins could not be reached by surface cleaning. We carried out the wash on the same visit. Applying a chemical solution under pressure across the coil face to dissolve and flush the mould from between the fins. The drain tray was scrubbed and flushed separately to remove the biofilm layer. After reassembly, we ran the unit and confirmed that the air coming from the vents was clean. We also suggested the shop owner run fan-only mode for a few minutes after switching off the cooling each day, and schedule a chemical servicing every six months given the shophouse humidity levels.

The musty smell was gone after the chemical servicing and had not returned after a week. No parts were replaced and the unit did not need swapping.

Musty smell: filter cleaning vs chemical servicing.

• •A general service cleans the filter and wipes accessible surfaces. Mould on the evaporator coil sits behind the filter, deep between fins spaced just 1-2mm apart. No cloth or brush can reach it. Only a chemical servicing with pressurised solution strips the fin surface clean.
• •Musty smell that returns within a few days of servicing almost always points to the coil, not the filter. The filter is a screen. If it is clean and the smell persists, the source is downstream. Ask your technician to open the blower housing and inspect the coil face before scheduling another general service.
• •Shophouse and ground-floor retail units are more prone to coil mould because doors open frequently, keeping ambient humidity high. The coil surface never fully dries between cycles, giving mould spores the persistent moisture they need to colonise.
• •Running the fan on its own for a few minutes after switching off the cooling helps dry the coil surface and slows mould growth between services. This is especially useful in high-humidity environments where the unit cycles on and off frequently.