AC smells from the vents: musty, dirty socks, burning, rotten egg or chemical
Classify the smell first because it decides whether you clean, repair or evacuate. Musty or dirty-sock odors are usually microbial growth on a wet coil, dirty filter or slow drain. Burning, electrical or 'gunpowder' odors point to an overheating motor, wiring or board: shut it down. Rotten egg can be a gas leak (evacuate) or a dead animal in the ducts. A sweet or chemical odor may be a refrigerant leak, but many refrigerants have little smell, so it is not a reliable test.
Before you start
- Rotten egg / sulfur smell near a gas furnace or gas appliance: do not operate switches, get everyone out, and call the gas utility or 911 from outside
- Burning or electrical smell: turn the system off at the thermostat and the breaker; if there is smoke, leave and call 911
- No refrigerant work: a suspected refrigerant leak is diagnosed and referred to an EPA 608 certified technician
Diagnostic steps
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