What is 'dirty sock syndrome'?

A musty smell caused by mold and bacteria on the evaporator coil. It's most noticeable when the AC first turns on. Professional coil cleaning resolves it.

Should I be concerned about a burning smell?

A brief burning smell when first using heat in fall is normal (dust burning off). Persistent burning smell indicates an electrical problem - turn off the system and call a technician.

Can Wait

Musty or Bad Smell from Vents

Unpleasant odors coming from your heating or cooling vents

Call (650) 396-3290 Book Service

Signs You Have This Problem

Musty or moldy smell when HVAC runs

Dirty sock syndrome smell

Burning smell from vents

Rotten egg smell (may indicate gas leak)

Chemical or electrical burning odor

What Causes This Problem

Mold or mildew in ductwork or on coils

Dirty evaporator coil (dirty sock syndrome)

Dead animal in ductwork

Clogged condensate drain

Electrical problems (burning smell)

Gas leak (rotten egg smell)

What You Can Check First

Before calling for service, try these simple troubleshooting steps:

Replace air filter

Check condensate drain for clogs

Look for visible mold around vents

Note when smell occurs (heating, cooling, both)

When to Call a Professional

If you smell gas (rotten eggs) or burning, turn off the system and call immediately. Musty smells often require professional coil cleaning or duct treatment.