Air conditioners should not smell like anything. When one starts to, the type of smell narrows the cause down quickly. Most are about moisture and dirt. A few are electrical or gas and need attention right away.
Leave now: rotten eggs or sulfur
Natural gas has an added odorant that smells like rotten eggs. An air conditioner does not use gas, but if your furnace shares the same air handler, a gas leak can be pulled through the ducts. Leave the house, take everyone with you, and call PG&E at 1-800-743-5000 or 911 from outside. Do not flip switches on your way out. Our article on furnace smells covers this in more detail.
Turn it off: burning or electrical smells
A sharp burning plastic or electrical smell from the vents or the outdoor unit can mean an overheating motor, a failing capacitor, or wiring insulation burning. Turn the system off at the thermostat and the breaker and call. Do not run it until it has been checked.
The exception is a dusty burning smell the first time you run the heat after months off. That is dust burning off the heat exchanger or heat strips, and it should fade within an hour.
Musty or moldy
The most common AC smell, and it usually means moisture is sitting somewhere it should drain away. Air conditioners pull water out of the air, and that water has to leave through the drain pan and condensate line.
- A clogged or slow condensate drain leaves standing water in the pan, where mold and bacteria grow.
- A dirty evaporator coil collects dust that stays damp and grows biological film.
- A dirty or damp filter.
- Mold in the duct work, especially flex duct in a hot attic where condensation forms on the outside or leaks let humid air inside.
- A system that short cycles never runs long enough to dry the coil. See an AC that keeps turning on and off.
Start by replacing the filter and flushing the condensate line with distilled vinegar. If the smell persists, the coil and pan need cleaning. If there is visible mold in the ducts, the EPA notes that insulated duct and duct board generally cannot be effectively cleaned and should be replaced. We cover this in is air duct cleaning worth it.
Vinegar or sour
A vinegar smell often comes from the same moisture problem as musty odors, bacteria in a dirty coil or drain pan. It can also come from an electric motor overheating, which some people describe as sour or chemical. If it comes with any burning note or the system is not running normally, turn it off and call.
Dirty socks
A specific, well known smell caused by bacteria growing on the indoor coil, often noticed when the system first starts after sitting. A professional coil cleaning usually fixes it, and keeping the drain clear helps prevent it coming back.
Exhaust or chemical smells
Car exhaust smell from vents can mean a furnace heat exchanger or venting problem if the furnace shares the ducts. Make sure your carbon monoxide alarm works and call. A sweet chemical smell near the unit can point to a refrigerant leak.
Can AC smells make you sick?
Mold and bacteria growing in a system can aggravate allergies and asthma for sensitive people. Gas and carbon monoxide are genuine hazards. Burning smells are fire risks. Musty smells are rarely an emergency, but they are worth fixing rather than masking with air fresheners.
What we do
We inspect and clean the coil and drain pan, clear the condensate line, check the ducts for leaks and mold, and test electrical components if the smell is burning. On any system sharing ducts with a gas furnace, we take a carbon monoxide reading. Our diagnostic is a flat $69 and it is credited toward the repair. See our indoor air quality services and duct work services.
Frequently asked questions
Why does my AC smell musty?
Usually moisture that is not draining away: a clogged condensate line, standing water in the drain pan, a dirty evaporator coil, a damp filter or mold in the ducts.
Why does my AC smell like vinegar?
Often bacteria in a dirty coil or drain pan. It can also come from an overheating motor. If there is any burning note or the system behaves oddly, turn it off and call.
What is the dirty sock smell from my AC?
A buildup of bacteria on the indoor coil, often noticed at startup. A professional coil cleaning usually resolves it.
My AC smells like burning. Is that dangerous?
A burning plastic or electrical smell can mean an overheating motor, capacitor or wiring. Turn the system off and have it checked. A brief dusty smell the first time the heat runs in fall is normal.
My AC smells like rotten eggs. What should I do?
Treat it as a possible gas leak. Leave the house, take everyone with you, and call PG&E or 911 from outside.
Can a musty AC make you sick?
Mold and bacteria can aggravate allergies and asthma for sensitive people. It is worth fixing the moisture problem rather than masking the smell.