You switch on the air conditioner after months off and a musty, sometimes mouldy, sometimes sour smell hits you immediately. That's not normal and it's not something to cover up with air freshener. What you're smelling is mould or bacteria growing inside the unit, and you're breathing it in. Here's what's causing it and how to actually clear it.
Why it smells when you first switch it on
Condensation forms inside the indoor unit whenever it's cooling. If that moisture sits without drying out, mould and bacteria develop on the filters and evaporator coil. When you restart the unit after a long break, the airflow sends everything that's built up straight into the room. The smell is strongest on the first day of summer operation and after particularly humid spells.
The most common causes
- Dirty filters. Dust and moisture trapped in the filters are ideal conditions for mould growth. This is the single most common cause of musty odour.
- Fouled evaporator coil. The coil fins collect dirt and condensate. Without cleaning they become a colony of bacteria.
- Blocked condensate pan or drain line. Standing water in the pan decomposes and produces an acidic, sometimes sulphurous smell.
- Burning plastic smell. This is a different type of problem entirely and needs immediate attention. It can indicate an electrical fault. Switch the unit off and have it inspected before using it again.
What you can do yourself
The good news is that mould odour is almost always solvable through cleaning.
- Wash the filters. Remove them, rinse under lukewarm water, let them dry completely in the air, then refit. Never put them back damp or you'll make it worse.
- Clean the visible coil fins. Use a sanitising spray designed for air conditioners. Apply to the coil with the unit switched off, following the product instructions.
- Use the self-drying function. Many units have a mode that keeps the fan running for several minutes after shutdown to dry out the interior. Use it every time.
- Let it ventilate before switching off. Run the unit in fan-only mode for ten minutes before shutting down. This dries residual condensate from the coil.
When you need a professional service
If you've cleaned the filters and coil and the smell keeps coming back, the contamination is deeper, in the inner coil and the cross-flow fan. A professional sanitisation service opens the unit and treats these components with appropriate products. It's worth doing at the start of the season, before intensive use begins, especially if the unit is a few years old and has never been professionally cleaned.
How to prevent it coming back
Clean filters every two to three weeks during peak summer use, always activate the self-drying function when shutting down, and book a professional sanitation once a year. If your unit is old and the smell persists even after a thorough clean, it might be time to look at a newer model with antibacterial filters and an auto-clean feature. Browse current options in our air conditioner range.