Why Does My Central Air Smell Like Rotten Eggs?

Because a central air conditioner actively pumps air, any smell that originates inside it is bound to be broadcast throughout the house. Rotten egg smell could have many sources. Whatever the source, it isn't a pleasant smell, and it it''s not one you should take lightly.

A Dead Animal

What to Do

• Turn off the air conditioner. 
  
• Disassemble the ducts, find the animal and remove it. You may need professional help to do this, since air conditioning ducts are rarely accessible.
  
• Have the ducts professionally cleaned. You need specialized equipment and expertise to do the job.
  

Mice, rats and even birds can seek shelter in air conditioning ducts in winter, and if one dies there, an overpowering odor is usually the result. The odor isn't exactly like rotten eggs, but that isn't as important as the need to do something about it.

Gas Smells from a Nearby Source

What to Do

• Turn off the air conditioner and leave it off until you find the source of the leak.
  
• Refrain from using any electrical or gas appliances or lighting any flames. If the smell is strong, avoid using any electrical devices at all, including lights. 
  
• Go outside until the leak has been found and stopped.
  
• Call the gas company or an HVAC service representative.

Both natural gas and propane are treated with mercaptan to give them a rotten egg odor. Suspect a gas leak if you can't find another reason for the smells. If you have a central air unit in the basement, the gas could be coming from the furnace and circulating through the cooling system ducts. It could also be leaking somewhere near an air intake vent -- perhaps from the stove or a room heater -- and getting sucked into the system. It's also possible that the system itself includes a heat pump that runs on gas.