Turn on the split after months and you get that musty smell, sometimes moldy, sometimes an acidic odor reminiscent of wet socks. This is not normal and should not be masked with deodorizers. That smell is a sign that something is growing inside the unit that you then breathe in. Let’s see where it comes from and how to really get rid of it.
Why it smells as soon as you turn it on
Condensation forms inside the indoor unit when the device cools. If the moisture remains there without drying, mold and bacteria grow on the filters and coil. When you turn the air conditioner back on after a period of inactivity, the airflow hits them and brings all that accumulated odor right to your face. It’s stronger in summer at the first startup and after humid days.
The most common causes
- Dirty filters. Dust and moisture trapped by the filters create the perfect environment for mold. This is the number one cause of the musty smell.
- Crusted internal coil. The fins of the heat exchanger collect dirt and condensation. Without cleaning, they become a colony of bacteria.
- Clogged drip tray and drain. Stagnant water in the tray rots and emits an acidic, sometimes egg-like smell.
- Burnt plastic smell. This is different and should be taken seriously: it may indicate an electrical problem. Turn it off immediately and have it checked.
What you can do yourself
The good news is that the moldy smell can almost always be solved with cleaning.
- Wash the filters. Open them, rinse under lukewarm water, let them dry completely in the air, and put them back. Never put them back wet, or you’ll make things worse.
- Clean the visible fins. Use a sanitizing spray for air conditioners, spraying it on the coil with the unit off, following the instructions.
- Use the drying function. Many models have a mode that runs the fan at the end of the cycle to dry the inside. Activate it to prevent mold formation.
- Ventilate before turning off. Leave the unit on fan-only mode for ten minutes before switching it off to dry any remaining condensation.
When professional sanitization is needed
If you have cleaned the filters and fins and the smell returns, the dirt is where you can’t reach it—deep inside the coil and drum fan. Professional sanitization disassembles and treats these parts with specific products. It’s worth doing at the start of the season, before heavy use, especially if the unit is a few years old and has never been thoroughly sanitized.
How to prevent it from coming back
Wash the filters every two or three weeks during peak summer, use the drying function every time you turn it off, and have a sanitization done once a year. If your split unit is old and the smell doesn’t go away even after deep cleaning, consider a new model with antibacterial and self-cleaning filters: among recent air conditioners, many have features that keep the inside dry and reduce the problem at its root.
Frequently Asked Questions
Why does the air conditioner smell like mold when I turn it on?
Because moisture accumulates inside the unit on the filters and coil, allowing mold and bacteria to grow. When you turn it back on, the airflow brings that smell to your face. It can be removed by cleaning the filters and sanitizing the fins.
Does the sanitizing spray for air conditioners work?
Yes, for surface odors. It should be sprayed on the coil with the unit off, following the instructions. If the smell returns shortly after, the dirt is deep inside and professional sanitization is needed.
I smell something burning from the air conditioner: is it serious?
The smell of burnt plastic or electrical odor is different from mold and should be taken seriously. Turn off the unit and the circuit breaker immediately, and have a technician check the system before turning it back on.