Musty smell after general service traced to mould on evaporator coil

What we found

We removed the filter and opened the blower housing to inspect the evaporator coil directly. In a humid ground-floor space, the coil is the most likely source when filter cleaning fails to resolve a smell.

• 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. The ground-floor retail environment kept humidity high enough for mould to thrive between the fins even when the unit was running. General servicing only cleaned the filter, which sits in front of the coil and does not touch the fin surface.

What we did

A chemical wash was needed to strip the mould from the coil fins and clean the drain tray. We carried out the wash, flushed the tray, and confirmed the coil was clear before reassembly.

The musty smell was gone after the chemical wash. The shop owner confirmed it had not returned after a week. No parts were replaced and the unit did not need swapping.

What we learned

Musty smell: filter cleaning vs chemical wash.

• A general service cleans the filter and exterior. Mould on the evaporator coil sits behind the filter and can only be reached by a chemical wash that strips the coil fins.
• Shophouse and ground-floor retail units are more prone to coil mould because of higher ambient humidity near street level.
• Musty smell that returns within a few days of servicing almost always points to the coil, not the filter. If the filter is clean and the smell persists, the coil is the next place to check.