Why Does My AC Smell Musty or Sour When It Turns On?

Common Causes of Mould and Sour Smells

During cooling, the surface of the evaporator coil is cold, causing moisture in the air to condense into droplets that drip into the drain pan and exit through the drain pipe. If dust, hair, and skin cells accumulate inside the unit, the damp environment provides ideal conditions for mould to grow.

Hong Kong summers are humid and ACs run for long stretches almost every day. The evaporator coil, fan wheel (blower), and drain pan are especially prone to building up moist, organic grime. The sour odour at start-up is exactly this contamination being blown out by the airflow.

• Evaporator coil: dust and moisture combine to form black deposits
• Fan wheel (blower): accumulated dust on blades releases odour when spinning
• Drain pan and drain pipe: standing water mixed with dirt encourages mould
• Air filter: first line of defence — once dust breaks through, it reaches the internals

The First Step You Can Take Yourself

Switch the unit off and unplug it. Carefully remove the air filter and inspect it. If it is thick with dust, rinse it under tap water from the clean side (the side facing inward) toward the dirty side (the side facing the room), then let it dry completely before reinstalling. Restoring proper airflow sometimes reduces mild odours on its own.

Keep in mind: cleaning the filter only addresses the outermost layer. Grime on the evaporator coil, deep inside the fan wheel, and in the drain pan cannot be removed by washing the filter alone. If the smell persists after the filter is cleaned, the contamination is deeper inside and a professional deep clean is needed.

When to Call a Technician

Arrange for a technician to inspect and clean the unit if: the smell continues for more than a day or two after cleaning the filter; the musty smell is accompanied by water dripping or noticeably reduced airflow; the unit has not had a deep clean in a year or more; or someone in the household has young children, elderly residents, or respiratory sensitivities.

A deep clean typically covers the evaporator coil, fan wheel, and drain pan, plus clearing the drain pipe. Whether the smell is completely eliminated depends on how much grime has built up and the overall condition of the unit. The technician should be honest with you about expected outcomes before starting.

Mould Smell Self-Check List

• Switch off and unplug the unit before any cleaning
• Remove the air filter and check how much dust has built up
• Rinse from the clean side toward the dirty side under running water
• Allow to dry completely before reinstalling (damp filter worsens mould)
• Restart the unit and observe whether the smell improves
• If the smell persists after a day or two, arrange a deep clean
• Note whether dripping or reduced airflow is present at the same time

Frequently asked questions

My brand-new AC smells on first use — is that normal?

A faint plastic or manufacturing odour during the first few hours of operation is normal and usually dissipates quickly. If the smell persists or has a distinctly musty quality, it may indicate a damp installation environment or residue in the drain pipe — contact the installation technician to confirm.