Short answer
If it smells like burning plastic, wires or smoke, set the thermostat to "Off" and call a technician. If you see smoke or the smell gets stronger, get out and call 911. A musty or mildew smell usually comes from a dirty filter, a clogged drain line or mold on the coil. Replace the filter, make sure the drain line is not backed up and call if the smell stays.
Safety first
A burning or electrical smell is not normal for an AC. Turn it off at the thermostat and do not run it again until a technician checks it. If you see smoke or flames, or the smell keeps getting stronger, get everyone out and call 911. Do not open the panels of the unit. If you smell rotten eggs near a gas furnace, leave and call the gas company from outside.
What to do when your AC smells
1A smell from the vents
Sniff near a vent while it runs. Burning, musty or dirty socks? Each one points to a different fix.
Read all the steps
- A smell from the vents. Sniff near a vent while it runs. Burning, musty or dirty socks? Each one points to a different fix.
- Burning smell? Turn it off and call. Set the thermostat to "Off" and call a technician. Smoke, or a smell that keeps growing? Get everyone out and call 911.
- Musty smell? Replace the filter. With the system off, slide out the filter. A gray or damp filter holds dust and mold. Put in a new one of the same size.
- Look for standing water in the drain. Take the cap off the drain line port by the indoor unit. Water standing near the top means a clog, and that is where mildew grows.
- Back on, fresh air. Set the thermostat back to "Cool" and give it an hour. If the musty smell stays, the coil or the ducts may need a cleaning.
- Still not fixed? Call an HVAC contractor Especially if: it smells like burning plastic, wires or smoke, even once.
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What each smell usually means
- Burning plastic or electrical: a motor, wire or part overheating. Turn it off and call.
- Musty or mildew: mold or algae on the coil, in the drain pan or on a damp filter.
- Dirty socks: bacteria on the indoor coil, often right when the AC starts. A technician can clean the coil.
- A dusty smell for a few minutes the first time you turn on the heat: dust burning off the furnace. It should fade.
- Rotten eggs near a gas furnace: a possible gas leak. Leave and call the gas company from outside.
What not to do
- Do not keep running an AC that smells like burning to see if it goes away.
- Do not spray air freshener or disinfectant into the vents or onto the coil.
- Do not mix bleach and vinegar in the drain line. Mixing cleaners can release toxic gas.
- Do not open the panels of the unit to look for the source.
When to call an HVAC technician
- It smells like burning plastic, wires or smoke, even once.
- The musty smell stays with a new filter and a clear drain line.
- You see mold on the coil, in the drain pan or around the vents.
- It smells like dirty socks every time the AC starts.
Frequently asked questions
Why does my AC smell like something burning?
An overheating motor, a failing part or a hot wire can smell burnt. Turn the system off and call a technician. If you see smoke, get out and call 911.
Why does my AC smell musty?
Usually mold or algae on the indoor coil, in the drain pan or on a damp, dirty filter. Replace the filter and check the drain line. If the smell stays, have a technician clean the coil.
Is it normal for the AC to smell when it first turns on?
A faint dusty smell for a few minutes at the start of the season can be normal. A smell that lasts, or any burning or electrical smell, is not.
Is a musty smell from the AC dangerous?
It is not an emergency, but it means moisture and mold in the system, which can bother people with allergies or asthma. Fix the cause instead of covering the smell.
Why does my AC smell like dirty socks?
Bacteria growing on the indoor coil, often called dirty sock syndrome. It is common in humid climates. A technician can clean and treat the coil.
Can I spray disinfectant into my AC vents?
No. Sprays only cover the smell for a while and can leave residue on the coil. Replace the filter, clear the drain and have the coil cleaned if the smell stays.