Why Does My Air Conditioner Smell Musty?

Written by Ofek Shamir, owner of The Guys Group

Why does my air conditioner smell musty?

A musty smell from an air conditioner almost always means moisture and organic build-up are sitting inside the indoor unit, most often on the cooling coil, in the fan barrel, in the drain tray or in a filter that hasn’t been cleaned. When the system cools, water condenses on those parts. Add dust and skin cells and you get the damp, dark conditions mould and bacteria grow in. The first blast of air when the unit starts carries that smell into the room. Cleaning the filter helps, but the smell usually comes back until the coil, fan and drain tray are properly cleaned.

Key Takeaways

• A musty or “dirty socks” smell is a sign of mould or bacteria inside the unit, not just dust.
• The usual sources are the evaporator coil, the fan barrel, the drain tray and a clogged filter.
• Washing the filter is worth doing, but it rarely fixes the smell on its own.
• NSW Health notes that for people with asthma, inhaling mould spores may cause an asthma attack.
• If the smell returns within days of cleaning the filter, the unit needs a deep clean.

Where does the musty smell actually come from?

An air conditioner pulls warm room air across a cold coil. Moisture in that air condenses on the coil and drips into a drain tray, then out through a drain line. That is normal. The problem starts when the parts that stay damp also collect dust, pollen and skin cells. Over months, they become a surface mould and bacteria can live on.

• Evaporator coil: the fins stay cold and wet while the unit runs, and trap fine dust that the filter misses.
• Fan barrel (blower wheel): the curved blades pick up a black, sticky layer that sits right in the airflow, so anything growing there is blown straight into the room.
• Drain tray and drain line: standing water in a tray that doesn’t drain fully is one of the most common sources of a sour smell.
• Filter: a loaded filter holds moisture and debris against the airflow.

In a ducted system the same thing can happen at the fan coil in the roof space and along the ductwork, and the smell reaches every room the system serves rather than just one.

Is a musty-smelling air conditioner bad for you?

A smell on its own doesn’t tell you how much mould is present, but it is a sign there is growth somewhere in the unit. NSW Health says health effects of mould exposure include a runny or blocked nose, irritation of the eyes and skin, and sometimes wheezing, and that people with asthma, allergies or other breathing conditions may be more sensitive to mould. For people with asthma, NSW Health notes that inhaling mould spores may cause an asthma attack. If anyone in the home has asthma or allergies, it is worth treating the smell as a reason to clean the unit sooner rather than later.

Will the musty smell go away on its own?

Usually not. Running the unit on “dry” or “fan” mode after cooling can help the coil dry out and reduce new growth, but it won’t remove build-up that is already there. Masking sprays and scented filters cover the smell for a short time without touching the cause.

What can you do yourself?

Always switch the unit off at the wall or isolator before touching anything inside it.

1. Wash the filter. Slide the filter out, rinse it with warm water and a little mild detergent, and let it dry completely in the shade before refitting it. A damp filter put straight back in adds to the problem.
2. Wipe what you can reach. Wipe the front panel, vanes and the outlet area with a mild detergent solution. NSW Health suggests mild detergent or a diluted vinegar solution (4 parts vinegar to 1 part water) for routine clean-up of mouldy surfaces.
3. Dry the unit out. After cooling, run the fan or dry mode for 20–30 minutes so the coil isn’t left sitting wet.

Avoid spraying water or cleaning products into the coil, fan or electrical parts. It pushes debris deeper, can leave the drain tray overflowing and can damage the electronics. The coil and fan barrel are the parts that usually cause the smell, and they can’t be cleaned properly without partly dismantling the unit.

What does a professional deep clean fix that a filter wash doesn’t?

A deep clean goes past the filter to the parts where the smell actually lives. On a split system that means the evaporator coil, the fan barrel, the drain tray and the drain line, cleaned with the right equipment while the wall, floor and furniture underneath are protected. On a ducted system it covers a new filter, the aluminium duct foils cleaned from the filter through to the return duct, a professional sanitise, and an airflow check and test run. It helps efficiency too. An energy.gov.au factsheet on HVAC coils explains that build-up on coils reduces the surface available for heat transfer and hurts system performance, and that biological growth on coils can degrade them to the point where they need replacing.

If the smell is one of several warning signs, see signs your split system needs a deep clean. For how often to book a clean, see how often to clean your air conditioner in Sydney.