When a building resident reports foul odors from air conditioning, facility managers have a wide range of potential culprits to consider. Often the odor itself provides a clue to the root cause of the problem and may indicate anything from a mild mold problem to a dangerous buildup of toxic fumes. 

Dirty Socks:

It’s an odor everyone knows—the stink of smelly feet. The root cause is often stagnant water which builds up in the condenser pan due to blocks in the pan or the condenser drain line. Combining stagnant water with dust and warmth provides an environment conducive to the growth of mold and mildew. The smell is usually strongest when the unit turns on and can be resolved by clearing clogs and cleaning the drainage pan.

A more problematic cause could be moisture in the ductwork if the ducts are not properly sealed or the unit is not removing moisture from the air properly. If so, professional cleaning of the ductwork is required, leaky ducts must be sealed, and the air conditioner serviced to ensure proper moisture removal.

A third potential cause of mildew odors—and the easiest problem to fix—is a dirty air filter. If you’re replacing filters regularly this usually won’t be an issue.


The smell of sewage suggests a sewer pipe crack or backed up sewage line. Gas from the sewage enters the ducts and spreads through the building. This is a potentially dangerous situation, as the gas in question is methane. Any sewage smells from HVAC systems should be reported to facilities managers or building operators immediately. 

  1. Exhaust Fumes:

    As an electric device, air conditioning units should not produce exhaust fumes. If such smells are present, they suggest refrigerant fluids may be leaking somewhere in the system. The fluid used in AC units can be harmful, and should only be handled by an HVAC technician certified in their use and removal.

  2. Rotten Eggs:

    Want to hear an HVAC technician groan? Report a smell of rotten eggs. The unpleasant odor suggests a small animal or bird made its way into the ductwork and died. Professional cleaning of the ductwork should solve the problem.

  3. Acrid, Smoky Odors:

    Hot metallic or gunpowder smells can indicate a short in a circuit board or unit fan. If the smell is more like burning plastic, something may have fallen on the unit’s coils or heat exchange and melted. A thorough HVAC service is the best solution. 

  4. Skunky Smells:

    Like sewage and exhaust fume odors, a skunky smell suggests a potentially dangerous situation. The smell suggests a possible gas leak occurring near the ductwork and requires immediate attention.


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