An air conditioner with a sour smell is likely suffering from an issue involving its condensation removal system. Condensation is a byproduct of the cooling method inside the air conditioner and, drained improperly, can create the conditions necessary for foul-smelling bacteria and mold to grow inside your system. There are a few things you can do to eliminate the source of the odor.
Condensation From Dehumidifying
All air conditioners are functionally similar; they pull in indoor air through a filter and blow it across cold copper coils filled with refrigerant. These coils simultaneously reduce the temperature of the air and "sweat," or accumulate condensation, from the humidity in the air. The condensation then drains into a condensate pan for disposal, while the cold dry air is returned to the home. The condensate pan is attached to a condensate pump, which pumps the water either to a floor drain or outside the home into the soil.
A clogged or old air conditioner filter can cause a host of problems for your system. A filter that is not functioning properly will prevent the air conditioner from pulling in sufficient air to cycle through the system. This results in the air inside the air conditioner increasing in humidity and making everything in the system damp, creating the conditions for mold and mildew growth. Ensure that your filter is functioning properly, and clean or replace the filter at the intervals suggested in the user manual for your unit.
If your air conditioner has been run for any length of time without a filter or has not been serviced recently, you may have a clogged condensate pan. The condensate pan, located in the air handler directly beneath the refrigerant coils, catches condensation as it drains from the cold coils. Hair and other debris can stop up the drain hole in the center of the pan, causing a water backup and foul odor. Wipe out the pan, and clean the pan drain by using a paper clip or piece of wire to clean out the first few inches of the drain opening.
Mold in Ducts
If you have had a moisture problem with your air conditioner for a prolonged period of time, you may now have mold and mildew growth in your ducts. This is a potentially dangerous situation, as some molds and mildew are quite damaging to indoor air quality. If you suspect your ducts to be the issue, replace them immediately, and contact a mold specialist for information on how to reduce the risk of inhalation.