An air conditioner smells from mold or bacteria on the coils or in drain water. Clean it, replace the filter, and call a technician if it persists.
Common causes
- Mold and bacteria on the coil.
- Clogged drain pan.
- Dirty filter.
- Dust burning on first use.
- Refrigerant leak (sweet smell).
- Electrical burning (a serious sign).
Fix
- Replace the filter every 1 to 3 months.
- Clean the evaporator coil and drain line.
- Run fan to dry out.
Safety
A burning smell means turn it off and call a technician.
Mold and mildew
Moisture in the evaporator coil and drain pan encourages mold and bacteria. A musty or “dirty sock” smell is common when the system starts, and is a sign of microbial growth. This is the most common cause.
Clogged condensate drain
A blocked drain line keeps water in the pan, causing stagnant, moldy smells and possibly water leaks.
Dirty filter
A dirty air filter holds dust and moisture. Replace filters every 1 to 3 months.
Other smells and what they mean
- Rotten egg or sulfur: possible natural gas leak (leave the house and call the gas company) or a dead animal. Natural gas has a sulfur like odorant added.
- Burning smell: dust burning off when heating season starts, but persistent burning may indicate electrical problems; shut it off.
- Chemical or sweet smell: refrigerant leak, which needs professional repair.
- Dead animal smell: a rodent in the ductwork.
- Vinegar smell: electrical issue or mold.
Cleaning steps
- Turn off power.
- Replace the air filter.
- Clean the drain line with a mixture of 1 cup bleach or white vinegar followed by water. Do not mix chemicals.
- Clean the evaporator coil with a coil cleaner.
- Run the fan for 10 to 15 minutes after cooling to dry out coils.
Prevention
Annual professional maintenance, UV lights in the air handler, and keeping humidity below 50% help.
Car air conditioners
A musty smell in a car A/C often comes from the evaporator and cabin filter. Replace the filter and use a cleaning spray.
Safety
Mold can worsen allergies and asthma. Call a technician for a persistent smell or suspected refrigerant leak.
Different smells, different problems
A musty or sour smell usually means mold or bacteria growing on a wet evaporator coil, a dirty filter or a clogged condensate drain that holds standing water. A burning smell can signal an overheated motor or electrical fault and the unit should be turned off. A smell like rotten eggs may indicate a natural gas leak near a gas furnace, so leave the house and call the gas utility from outside. A smell like sewage can come from a dry drain trap or plumbing issue. A chemical or refrigerant smell may indicate a leak.
Cleaning and prevention
Check the air filter monthly and replace it every 1 to 3 months, as recommended by the Department of Energy. Pour a cup of white vinegar down the condensate drain line to prevent algae. Have a technician clean coils and service the system yearly. A professional can also check ducts for mold. In cars, a musty AC smell often comes from the cabin air filter or evaporator, so replace the filter about every 12,000 to 15,000 miles and run the fan with AC off for a minute before shutting the car.
Based on data from the US Department of Energy and the EPA
