Walk into your bedroom after a long day, switch on the aircon, and — a stale, musty smell hits you within seconds. If you live in Singapore, this is annoyingly common. The combination of near-constant humidity, high outdoor temperatures, and the fact that your aircon pulls in moisture-laden air means mould thrives inside units far faster here than in drier climates.
Here's what's actually happening, why it matters for your health, and the real fixes — not the band-aid solutions some service companies will try to sell you.
Why Aircons Get Mouldy in Singapore
Singapore's relative humidity averages 80–90% year-round. Every time your aircon runs, the evaporator coil condenses moisture from the air. That water should drain away through the condensate line. But when it doesn't fully drain — or when moisture lingers in the internal foam insulation, fan blades, or drainage tray — mould colonies establish within weeks.
Common causes specific to Singapore homes:
- Drainage tray sitting flat or tilted wrong — very common in older HDB BTO installations where units were mounted quickly
- Clogged condensate drain line — dust and biofilm build up, especially in units above kitchens or near cooking areas
- Units left off for extended periods — if you travel frequently or use the aircon only at night, daytime warmth and humidity encourage mould growth in the idle unit
- Dirty filters left uncleaned — blocked airflow means more condensation, less evaporation, and moisture stays trapped inside
- Internal foam insulation deteriorating — older units (5+ years) often have insulation that crumbles, trapping moisture in porous material
Health Risks You Shouldn't Ignore
A mouldy aircon isn't just unpleasant — it's a genuine health concern, especially for vulnerable people.
Mild symptoms:
- Runny or stuffy nose
- Throat irritation
- Headaches that improve when you leave the room
- Skin irritation or unexplained rashes
More serious risks for sensitive groups:
- Asthma triggers — mould spores are a known asthma trigger, and Singapore's National Environment Agency (NEA) has flagged indoor air quality as a public health concern
- Allergic reactions — particularly for children and elderly family members sharing the same HDB flat
- Chronic sinus issues — prolonged exposure can lead to persistent sinus inflammation
Studies from the Singapore General Hospital respiratory department have linked prolonged exposure to indoor mould spores with increased rates of allergic rhinitis — and the aircon is the primary delivery system for circulating those spores throughout your home.
Quick Fixes You Can Do Today
Before calling a professional, try these steps. They won't solve a deep mould problem, but they'll help with surface-level smells:
- Clean or replace filters — remove the front panel, pull out the mesh filters, wash with warm water and mild detergent, dry completely before reinserting. Do this every 2–4 weeks in Singapore's humidity.
- Run the fan-only mode for 30 minutes — after switching off cooling, leave the fan running to dry out internal moisture. Many people skip this step, and it's one of the easiest mould preventers.
- Spray the coils with coil cleaner — available at most hardware stores in Singapore (HomeFix, Selffix). Turn off the unit, spray the evaporator coil gently, wait 15 minutes, then turn on to drain. Not a permanent fix, but it helps.
- Check and clean the drainage tray — if you can access it (usually by lifting the front panel), wipe it down and check that water flows freely through the drain line.
When a Basic Service Won't Cut It
If the smell persists after the above steps, your aircon likely needs a chemical overhaul, not just a standard service.
A basic service cleans filters and checks drainage — it does NOT deep-clean the internal components where mould hides. A chemical overhaul involves:
- Full disassembly of the indoor unit
- Chemical wash of the evaporator coil, fan blower, and drainage tray
- Removal of biofilm and mould from internal surfaces
- Anti-mould treatment applied to foam insulation
Cost in Singapore: $120–$180 per unit for a chemical overhaul. A basic service is $60–$90 but won't fix deep mould issues.
For a full breakdown of when to choose which service, see our guide: Aircon Servicing vs Chemical Overhaul: Which Does Your HDB Need?
Prevention: Keep Mould From Coming Back
The cheapest fix is prevention. In Singapore's climate:
- Clean filters every 2–4 weeks — non-negotiable in tropical humidity
- Run fan-only mode after each cooling cycle — 15–30 minutes dries out internal moisture
- Keep the unit running occasionally — if you're away for a week, have someone turn it on periodically, or at minimum leave windows open to reduce ambient humidity
- Schedule a chemical overhaul every 3–4 years — or sooner if you notice early warning signs
- Check drainage annually — pour a cup of diluted bleach through the drain access point to kill biofilm
How Much Does It Cost to Fix?
Quick summary of typical Singapore pricing (2026):
- Filter cleaning (DIY): Free
- Standard aircon service: $60–$90 per unit
- Chemical overhaul: $120–$180 per unit
- Drainage line flush: Usually included in service calls
- Fan blower replacement (if damaged by mould): $80–$150
For a complete pricing guide, see: How Much Does a Plumber Cost in Singapore in 2026? — while focused on plumbing, the page includes general home service cost benchmarks.
When to Call a Professional
Call an aircon technician if:
- The smell returns within days of cleaning filters
- You can see visible mould (black or green patches inside the unit)
- Family members are experiencing respiratory symptoms
- The unit is more than 5 years old and has never had an overhaul
- Water is leaking from the indoor unit
For tips on choosing a reliable service provider, read: How to Vet a Home Services Provider in Singapore: 7 Questions