It’s usually the same moment every spring: the first warm day rolls in, you switch the AC on for the first time in months, and instead of cool clean air you get a blast of something that smells like a damp gym bag. If that’s happening to you right now, you’re not imagining it, and you’re not alone — it’s one of the most common calls we get this time of year.
The short answer is that a musty smell almost always means moisture and mould somewhere inside the system. The longer answer is worth understanding, because the fix — and whether it’s a five-minute job or a technician visit — depends on exactly where that moisture is sitting.
| 🗣️ Insight A brief musty smell in the first minute of the season’s first run is genuinely normal — dust and a little condensation sitting in the unit over winter gets pushed out as soon as air starts moving. It’s when the smell lingers past that first run, or comes back every time the unit starts, that it’s telling you something. |
1. Is a Musty Smell Actually Normal?
A faint musty or dusty smell for the first few minutes of the season’s first run is common and usually nothing to worry about — it clears as the built-up dust and light condensation gets cycled out. What isn’t normal is a smell that persists for more than a day of use, comes back every single time you switch the unit on, or gets stronger over the run rather than fading. That pattern points to active mould or standing water somewhere in the system rather than just leftover dust.
2. What’s Actually Causing the Smell
The mechanism is almost always the same: your AC removes humidity from the air as it cools, and that moisture has to go somewhere. If it doesn’t drain or dry out properly between cycles, it sits in a dark, enclosed space — exactly the conditions mould and mildew need to grow. Where that happens depends on your system type.
Split Systems: The Coil and Fan Barrel
This is the most common source we find. If the indoor coil and fan barrel stay damp and dusty between uses, a thin microbial film builds up on the surface. Every time the fan starts, that smell gets pushed straight into the room — which is exactly why it’s often strongest right when you switch the unit on.
Ducted Systems: The Drain Tray and Return Air Path
With ducted systems, the smell often comes through several vents rather than one obvious spot, which makes it trickier to pin down. It usually traces back to the drain tray near the indoor unit, or to moisture sitting in the return air duct itself.
Other Common Sources
- Dirty filter — A filter clogged with dust holds moisture against it, giving mould something to grow on right in the airstream.
- Blocked condensate drain — If the drain line is clogged, water backs up into the drain pan instead of draining away, and standing water is exactly what mould needs.
- Leaky or poorly sealed ductwork — Lets humid air into the duct system where it condenses, particularly noticeable in older homes.
| ⚠️ Warning Don’t spray retail air fresheners, disinfectants, or any liquid directly into the unit or vents to mask the smell. Most household sprays aren’t rated for use near electrical components or coils, and they can damage the unit or simply push the smell around without fixing the mould causing it. |
3. What You Can Safely Try Yourself First
A few checks are safe to do without any tools or licence, and they solve the problem more often than you’d expect:
- Clean or replace the filter — This alone resolves a large share of musty-smell complaints
- Check the drain pan (usually visible on the underside of a split system’s indoor unit) for standing water
- Run the fan-only mode for 20-30 minutes after cooling to help the coil dry out fully between uses
- Make sure nothing is blocking airflow around the indoor unit or the return air grille
If you haven’t checked your filter type recently, our guide on reusable vs disposable air filters covers which one you likely have and how to clean or swap it properly.
| 💡 Pro Tip Get into the habit of running fan-only mode for half an hour after the AC finishes a cooling cycle, especially in humid weather. It dries out the coil before mould has a chance to establish itself — one of the simplest ways to prevent this smell coming back at all. |
4. When Does This Mean I Need a Professional?
Book a technician if the smell persists after cleaning the filter and checking the drain pan, if it’s coming from multiple vents on a ducted system, or if it’s been building up for more than one season. At that point the mould is usually established on the coil itself, which needs an antimicrobial treatment and proper coil clean — not something a filter change alone will fix.
A typical case we see: a customer runs their split system through a humid summer without ever using fan-only mode to dry it out afterward. By the following spring, the musty smell isn’t just present at start-up anymore — it’s there the whole time the unit runs. By then, the mould has spread across the coil and fan barrel, and a filter clean makes no difference at all. A same-day service call resolves it, but it’s the kind of job that’s far simpler (and cheaper) caught six months earlier.
5. Is It Always Mould? Other Smells to Know
Musty isn’t the only odour worth knowing about, and it’s worth telling them apart:
- Musty / damp sock smell — Mould or mildew, as covered above.
- Sour or vinegar-like smell — Usually stagnant water in the drain pan or line, similar cause to musty but further along.
- Burning or electrical smell — Switch the unit off immediately and call a technician. This points to an electrical fault, not moisture, and isn’t safe to keep running.
- Rotten egg or sewage smell — Often a dry P-trap allowing sewer gas to be drawn in through the drain line, rather than an issue with the AC itself.
6. Why This Is Worse in Sydney Specifically
Sydney’s humidity means evaporator coils are handling more moisture than they would in a drier climate, which is exactly the condition mould needs. It’s also why the smell tends to show up strongest at the very start of the season — a system that’s sat switched off through a humid Sydney winter has had months for any residual moisture to sit undisturbed before you switch it back on.
7. How Do I Stop This Happening Every Year?
Prevention comes down to the same few habits: clean the filter regularly, use fan-only mode to dry the coil after cooling, and get the system professionally cleaned once a year — ideally before summer, which is also the best time to catch anything else that’s crept up over winter. If you haven’t done that check yet this year, it’s worth running through it before the weather turns.
Quick-Glance Checklist
- Faint smell in the first few minutes only? Normal — no action needed
- Smell persists past the first run? Clean or replace the filter
- Check the drain pan for standing water
- Run fan-only mode for 20-30 minutes after cooling
- Smell from multiple vents, or been building for a season? Book a professional coil clean
- Burning smell? Switch off immediately and call a technician
Bottom Line
A musty smell is your AC telling you moisture is sitting somewhere it shouldn’t. Caught early — a filter clean, a dry-out cycle — it’s a five-minute fix. Left alone, it becomes a coil full of mould that needs a professional clean to sort out.
Smell not going away after the basics?
Book a split system clean and we’ll sort out the source, not just the symptom.