Aircon Anatomy

Where Mould Hides — Drip Pan, Blower, Evaporator

Most homeowners assume cleaning the filter handles aircon mould. The filter is the LEAST important site. The mould you smell and inhale lives in three places you can't see without taking the unit apart. Here's the anatomy.

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Site 1 — Drip pan + drainage line

The drip pan sits under the evaporator and catches condensation. In a well-installed unit it drains continuously through a sloped pipe. In reality, 60% of units we open have standing water in the pan from a partly blocked drain. That water grows biofilm — bacteria first, then mould — which the running fan aerosolises into the room. The drainage line itself, especially the bend before exiting the wall, is where biofilm grows densest.

Site 2 — Blower wheel (squirrel-cage fan)

The blower wheel is a cylindrical fan with dozens of curved blades. It spins constantly when the unit runs, coated in a film of moisture and dust. Mould on the blower is the dominant source of the constant musty smell. Cleaning the blower requires removing it from the housing — a 20-minute job that adds maybe $20 of labour but transforms the unit. It's the single biggest difference between a $50 wash and a $90 hygiene service.

Site 3 — Evaporator fin coil

The cold aluminium fins behind the filter. Visible black streaks here are advanced colonisation. Treatment must be non-corrosive — acidic washes corrode the fins and shorten unit life by 30–40%. We use non-corrosive steam + a non-residual cleaning solution; the fins come out clean without damage.

Site 4 (minor) — The front filter

Yes, the filter does collect spores. Washing it monthly is good hygiene. But filter mould is a downstream symptom of the three sites above, not the source. Cleaning only the filter is like wiping the floor while the tap is running.

Site 5 (rare but serious) — Inside the wall ducting

For centralised systems or units with long ducting runs, mould can establish inside the duct itself. Diagnosis requires a borescope. Treatment is more involved and costs from $250 above standard hygiene. We flag it on the first visit when present.

For surface mould you can already see, jump straight to professional mould removal in Singapore. For the full topic, see the Aircon Mould Singapore pillar.

Frequently Asked Questions

Can I see mould on the blower wheel myself?

Not without removing the front cover and the airflow guide vane below the evaporator. We're happy to show you on the first service visit — many customers ask, and seeing the contrast between cleaned and uncleaned blades is convincing.

Does the indoor unit need to be removed from the wall?

No. All mould sites are accessible with the unit in place — front cover off, evaporator and blower exposed. We use containment plastic to catch runoff. The full service is done on the wall.

How do I know if my drainage line is partly blocked?

Listen for gurgling when the aircon first switches off. Water dripping inside the unit (not flowing out cleanly) confirms partial blockage. We clear the line as part of every service.

Get every site cleaned — $90/unit hygiene service covers all three sites plus antimicrobial coating.

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