Unplug the unit immediately. A burning smell is a potential fire hazard. The cause is usually dust accumulation on internal electrical components or the motor.
- With the unit unplugged and completely cool, remove the grill and both filters.
- Inspect the empty cavity for any visible debris, dust bunnies, or foreign objects near the fan or motor housing.
- Use a vacuum with a hose and brush attachment to carefully remove all loose dust from inside the unit. Do not use compressed air, as it can drive dust deeper into electrical parts.
- Reinstall only clean, undamaged filters. Plug the unit in outdoors or in a well-ventilated area and turn it on to see if the smell recurs.
If that didn't work, and the burning smell returns with clean filters and a clean interior, the fan motor or wiring is overheating and failing. Do not continue to use it.
Heads up: This smell is a major warning sign. If thorough internal cleaning doesn't resolve it, the unit is unsafe and must be taken out of service.