Persistent musty smell: mould deep inside the cooling coil
The unit had been serviced twice in two months, but the musty smell came back within days each time. The filters were clean at every visit. That ruled out the obvious culprit and pointed somewhere a general service never reaches: the cooling coil.
By Team Snowflake | Reviewed 11 Mar 2026
Case summary
Panasonic Wall-mounted4 years oldShophouseRochor, Singapore
- Concern
- The shop owner worried the smell meant something was rotting inside the unit, or that the refrigerant was leaking.
- Found
- Mould colony established on evaporator coil fins, not reachable by general service filter cleaning
- Key check
- Removed filter and blower housing to inspect the coil face. Found visible mould growth across the fin surface
- Result
- 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.
What we were told
The unit in a small retail shop had a musty, damp smell for weeks. The regular contractor had cleaned the filters twice. The smell went quiet for a day, then returned each time. Replacement had been suggested as the next step.
What we checked
We removed the filter, clean from the recent service, and opened the blower housing for a clear view of the coil face. Two filter cleans had not fixed the smell, so the coil was the likely source in this humid ground-floor shop. We also checked the drain tray. Stagnant water with biofilm gives off a similar musty odour and is easy to miss.
The filter was clean, recently washed by the previous contractor.
The coil fins showed grey-green mould across the upper third of the surface.
The mould sat deep between the fins, where airflow could not dry it out.
The drain tray held a thin biofilm layer adding to the smell.
What we found
Mould had taken hold deep between the cooling coil fins. The coil runs cold and pulls moisture from the room air, which collects on the fins and drains into the pan below. In a ground-floor shop with the front door open all day, the air stays humid enough that the fins never fully dry between cooling cycles. That standing damp gave mould a place to grow in the gaps where little air moves. The general service only cleaned the filter, which sits in front of the coil and never touches the fins where the mould was growing.
What fixed it
We recommended a chemical servicing instead of another general service, since surface cleaning could not reach the mould between the fins. We did the wash on the same visit, flushing a chemical solution under pressure across the coil face to clear the mould from between the fins. The drain tray was scrubbed and flushed separately to remove the biofilm. After reassembly, we ran the unit and confirmed the air from the vents was clean. We suggested the owner run fan-only mode for a few minutes after switching off the cooling each day, and book a chemical servicing every six months given the shop's humidity.
Outcome
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.
What this case teaches us
A musty smell that survives a filter clean lives on the coil
- A general service cleans the filter, which sits in front of the coil. It never touches the coil fins where mould takes hold.
- If the smell returns within days of every service, the source is deeper than the filter. That points to the coil, not a failing unit.
- Ground-floor shops with doors open to humid air rarely let the coil dry out. A chemical servicing every six months keeps mould in check.
Related reading
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