Mould Spots or Dirty Vents on Your Aircon?

Mould growth and discolouration on your aircon vents usually point to internal moisture imbalance, poor airflow, or incomplete cleaning. Surface wiping won’t fix it — because the source is deeper inside the unit.

What Causes Mould or Stains Around Vents?

1. Biofilm on coil and blower

When moisture lingers inside the coil or blower chamber, bacteria and mould begin to grow. The spores eventually travel through the airflow path — and settle on the vents.

2. High humidity and weak airflow

Singapore’s humidity feeds microbial growth. When airflow is restricted (due to dirty filters or coil), moisture lingers — and mould accelerates.

3. Dirty or uncleaned blower fan

The fan blades inside the indoor unit are rarely cleaned in general servicing. Over time, they build up grime and shoot it through the vent when spinning.

4. Trapped condensation in trunking

If drainage is slow or insulation is poor, trapped moisture can affect the back of the vent — creating staining from behind.

5. Incomplete servicing

Filter-only cleaning leaves the blower, coil, and drain pan untouched. That allows internal bacteria to keep recirculating into the room.

What You Can Check First

• Are the stains dry or slimy?

• Any smell when unit first turns on?

• When was the last chemical or deep service?

• Any visible black spots along vent blades?

Photo evidence helps. If mould is visible, it’s already past surface level.

Why It Matters

Visible mould = active growth inside.

If ignored:

• Spores circulate into the room and fabric

• Respiratory irritation or allergies may worsen

• Coil efficiency drops due to biofilm

• Superficial cleaning won’t stop the source

Treating the symptom isn’t enough. The root cause lives inside.

When to Book a Diagnostic

✅ Black or grey marks on vent or blades

✅ Cleaning doesn’t remove the smell

✅ Mould returns quickly after normal servicing

✅ Odour strongest at startup or after downtime

We’ll inspect coil condition, blower fan, trunking moisture, and airflow health — and advise if a full chemical treatment is needed.