If you’ve just turned on your split system for the first time this Autumn — and something smells off — you’re not alone. Every year, thousands of Sydney homeowners fire up their heating and get hit with a weird, unpleasant smell coming straight from the unit.
The good news? It’s not normal. And it’s fixable.
In this guide, we’ll break down exactly why your split system aircon smells when heating, what each type of smell means, and what you should do about it — before it gets worse.
Why Does My Split System Smell When I Turn On the Heating?
The most common reason is simple: Your unit is dirty.
During Sydney’s warmer months, dust, moisture, mould, and bacteria build up inside your split system. When you switch to heating mode, the warm air passes through all that built-up gunk — and pushes the smell right into your room.
“A split system that smells bad isn’t just unpleasant — it’s circulating contaminated air through your home every time it runs.” — Real Time Air Duct Cleaning, Sydney
But not all smells are the same. Different odours point to different problems. Let’s break them down.
Why Does My Split System Smell Musty?
This is the most common complaint we hear from Sydney homeowners.
A musty smell means mould or mildew has grown inside your unit — usually on the evaporator coil, the filter, or inside the drainage tray. Sydney’s humidity during Autumn creates the perfect environment for mould to thrive, especially when the unit hasn’t been used for a few months.
What to do:
- Don’t ignore it. Mould spores spread through the air you breathe.
- Clean or replace the filter immediately.
- Book a professional split system cleaning — mould on the coil can’t be removed with a basic wipe-down.
⚠️ Warning: Running a mouldy split system for weeks can trigger respiratory issues, especially for kids, elderly people, and anyone with asthma or allergies.
Why Does My Split System Smell Like Fish?
A fishy smell from your split system is a serious warning sign — and it has nothing to do with actual fish.
This smell usually means an electrical component is overheating. Plastic parts, wiring insulation, or circuit boards can emit a fishy or burning plastic odour when they get too hot.
What to do:
- Turn off the unit immediately.
- Do not run it again until a technician checks it.
- This could be a fire hazard if ignored.
🔴 Warning: A fishy smell from any electrical appliance should never be ignored. Switch it off and call a professional straight away.
Why Does My Split System Smell Like Urine?
This one surprises people — but it’s more common than you’d think, especially in Sydney homes near trees or gardens.
A urine-like smell usually means rodents, possums, or birds have nested near or inside your outdoor unit and left droppings or urine behind. When the system runs, it pulls that contaminated air through the unit and into your home.
The other cause? Refrigerant leak. Some refrigerant gases have a sharp, ammonia-like smell that people often describe as urine or cat spray.
What to do:
- Inspect the outdoor unit for any signs of nesting or animal activity.
- If you suspect a refrigerant leak, call a licensed technician — refrigerant gases can be harmful.
💡 Pro Tip: In Sydney, possums are protected wildlife. If they’ve nested near your outdoor unit, you’ll need a licensed professional to handle removal properly.
Why Does My Split System Smell Like Burning or Smoke?
If your split system smells like burning when you first turn on the heating — a brief, mild smell is sometimes normal. Dust that settled on the heating elements burns off. This usually disappears within 5–10 minutes.
But if the burning smell continues or gets stronger, that’s a red flag.
Possible causes:
- Dust heavily clogged inside the unit
- Overheating motor or fan
- Electrical fault
What to do:
- If it lasts more than 10 minutes, turn off the unit.
- Have it inspected before running it again.
Why Does My Split System Aircon Smell Like Dirty Socks?
Dirty sock syndrome — yes, that’s the real name — happens when bacteria and mould grow on the evaporator coil. The warm, damp environment inside the unit is basically a breeding ground.
This smell is extremely common in Sydney because of the city’s humidity levels, especially heading into winter when systems sit unused for weeks.
The fix?
A proper coil cleaning and sanitisation by a professional. It’s not something a filter clean alone will solve.
How Often Should Sydney Homeowners Service Their Split System?
💡 Pro Tip: In Sydney’s climate, we recommend a full split system service at least once a year — ideally in Autumn before the heating season begins. If you run it heavily in summer too, twice a year is better.
A proper service includes:
- Filter cleaning or replacement
- Coil cleaning (evaporator + condenser)
- Drain tray flush
- Sanitisation and deodorising
- Full system check
This is what actually removes the smell — not just masking it with a spray.
Can a Smelly Split System Affect Your Health?
Absolutely. Here’s what you could be breathing in when your unit smells:
- Mould spores — linked to respiratory issues, sinus problems, and allergies
- Bacteria — can worsen asthma and trigger infections
- Dust and allergens — especially harmful for children and elderly
- Refrigerant gases — toxic at high concentrations
Sydney families spend a lot of time indoors during the cooler months. A dirty, smelly split system means poor indoor air quality every single day.
FAQ — Split System Smell Questions Answered
Q1: Is it normal for a split system to smell when first turned on?
A very mild, brief dust smell for the first few minutes is normal. Any strong or lasting smell is not normal and needs attention.
Q2: Why does my split system smell worse when heating than cooling?
Heating mode runs at higher temperatures, which activates and releases smells from mould, bacteria, and dust built up inside the unit.
Q3: How do I get rid of the musty smell from my split system?
A professional split system cleaning — including coil and drain tray sanitisation — is the only proper fix. Filter cleaning alone won’t remove deep mould.
Q4: Can I clean my split system myself to remove the smell?
You can clean the filter yourself. But coil cleaning, drain flushing, and sanitisation require professional equipment and shouldn’t be DIY’d.
Q5: Why does my split system smell like urine but I don’t have pets?
It could be a refrigerant leak or animal nesting near your outdoor unit. Both need professional inspection.
Q6: How long does a split system cleaning take in Sydney?
A standard professional split system clean takes approximately 1–2 hours depending on the unit size and how dirty it is.
Get Your Split System Smelling Fresh Again — Call Real Time Air Duct Cleaning
If your split system smells bad when heating, don’t just open a window and hope it goes away. The smell is a symptom — and ignoring it makes the problem worse.
Real Time Air Duct Cleaning provides professional split system cleaning across Sydney. Our team removes mould, bacteria, and built-up grime so your system runs clean, smells fresh, and keeps your family breathing healthy air this winter.
📞 Call us today and book your split system service.
Real Time Air Duct Cleaning — Sydney’s trusted air duct and split system specialists.
