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A musty or mildew smell usually comes from moisture sitting where it shouldn't, such as a dirty evaporator coil, a slow-draining pan, damp ductwork or a wet filter. A burning or electrical smell is different. It can mean an overheating motor or wiring, so turn the system off and call. A rotten-egg smell isn't the AC at all. Leave the house and call the gas utility.
Turn the system off and call a technician if:
Musty smells are far more common than burning ones. Pick a cause to see who can deal with it.
Common
Technician fixThe indoor coil is wet whenever the AC runs. In a humid climate, dust on that wet surface becomes a home for mold and bacteria, and the system blows the smell through the house, often strongest at startup.
Common
You can check thisWater that sits in the drain pan instead of draining away goes stagnant. A partly clogged line is common here because algae grows inside it.
Common
You can check thisA filter loaded with dust that gets damp can smell musty on its own. It's the cheapest thing to rule out.
Occasional
Technician fixIf the return side of the duct system leaks, it pulls in air from the attic, a wall cavity or the garage, and their smells come with it.
Occasional
Technician fixA burning, hot-plastic or electrical smell can come from a blower motor overheating, a failing capacitor, or wiring that's overheating at a loose connection.
Musty odors almost always come from somewhere water sits. In an air handler, that's this path.
For a musty smell only. For a burning or electrical smell, skip these and turn the system off.
Done all of these and it’s still not right?
That points to a part that needs a technician’s meter, not another reset.
A musty smell from the vents is a moisture problem, so it lasts as long as the moisture does. A stronger filter, a scented insert or a can of spray in the return can mask it for a few days, but EPA is blunt about the limits: portable air cleaners and HVAC filters "do not address the cause of mold and musty odors," and if the conditions that caused growth aren't corrected, "mold growth will recur."
The lasting fix is to keep the drip pan clean and the drain line flowing, then clean what has already grown. It's the same advice EPA gives for any building. In South Florida that usually means the drain line, the pan and the coil, and sometimes return leaks that pull damp attic or wall-cavity air into the system.
If you can see mold on the air handler or around the vents, don't scrub it yourself. EPA's guidance is not to run a system you know or suspect is contaminated with mold, because it can spread through the house. Have it looked at first.
A technician inspects the coil, drain pan and drain line, the blower and the first sections of supply and return duct. For a burning smell, the electrical components are checked for heat damage first. The technician explains the cleaning or repair needed and whether air-quality measures make sense. Free service call with repair.
That pattern usually points to growth on the indoor coil or standing water in the drain pan. When the system sits off, the damp surfaces build up odor, and the first blast of air carries it out. Cleaning the coil and making sure the pan drains usually solves it.
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