What are the signs a split system needs cleaning?
The clearest signs are a musty or sour smell when the unit starts up, weaker airflow than it used to have, longer run times to reach the same temperature, visible dust or black speckling on the vanes, and water dripping inside. The smell usually means biological growth on a cooling coil that stays damp; NSW Health notes that inhaling mould spores may trigger an asthma attack. Weak airflow and longer run times point at a loaded filter or a dirty coil. Dripping usually means a blocked drain tray or line. Any one of these on its own is worth a clean; two or more together means it is overdue.
Key Takeaways
- A musty smell, weak airflow and visible mould are the most common signs of a dirty split system.
- Rising energy bills without more use can point to a unit working harder than it should.
- Water leaking from the unit is a sign to book a clean sooner rather than later.
- New or worsening allergy symptoms at home can sometimes trace back to a dirty aircon.
Your split system doesn’t always announce it needs cleaning outright — but there are usually warning signs if you know what to look for. Here are seven to watch for.
1. Musty or Mouldy Smell When It’s Running
If your unit gives off a musty or mouldy smell as soon as it kicks on, that’s a strong sign mould or bacteria has built up inside. This is one of the clearest and most common signs a split system needs a deep clean, and it tends to get more noticeable the longer it’s left.
2. Weak or Uneven Airflow
If air coming out feels weaker than it used to, or airflow seems uneven across the room, buildup inside the unit is often restricting proper airflow. This usually gets gradually worse rather than appearing suddenly, so it’s easy to miss until it’s fairly advanced.
3. Visible Mould or Dust on the Front Panel or Vents
Take a look at the vents and front panel — visible dust, black spots or mould are a sign that what’s inside is likely worse than what you can see. If it’s visible on the surface, it’s typically built up internally too.
4. Rising Energy Bills Without Increased Use
A dirty unit has to work harder to heat or cool the same space, which shows up as higher energy bills even without any change in how much you’re using it. If your bills are creeping up and your habits haven’t changed, buildup is a common culprit.
5. Water Leaking or Dripping From the Unit
A blocked drain line — often caused by buildup — can cause water to back up and leak from the unit instead of draining properly. This is worth addressing quickly, since ongoing leaks can lead to further damage.
6. It’s Been More Than 6-12 Months Since the Last Clean
Even without obvious symptoms, buildup happens gradually. If it’s been over 6 to 12 months, it’s worth booking a check regardless of how things seem on the surface.
7. Someone in the Home Has New or Worsening Allergy Symptoms
If allergy or respiratory symptoms have appeared or gotten worse at home, a dirty aircon circulating dust, mould spores or bacteria can be a contributing factor. It’s worth ruling out, especially alongside any of the other signs on this list.
What Happens If You Ignore These Signs?
Left untreated, buildup keeps compounding — airflow gets weaker, energy bills keep climbing, and air quality keeps declining. None of it fixes itself, and it typically means a more involved (and pricier) clean the longer it’s left. For the full picture on how split and ducted systems compare, see split system vs ducted comparison, and for more on dirty airflow specifically.
Book a Split System Deep Clean
Spotted one or more of these signs? It’s worth booking a proper deep clean sooner rather than later. Explore our full air conditioning cleaning Sydney service, or get in touch for a free, no-obligation quote.
The musty smell is worth taking seriously on its own terms. NSW Health advises that inhaling mould spores may trigger an asthma attack, and the National Asthma Council Australia lists high-efficiency air filters — including those built into air conditioning — as part of managing mould and allergy triggers indoors. A cooling coil that stays damp is a place mould is happy to sit.
