A sour smell emerging from your air conditioning vents usually indicates an issue with the 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.
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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 gravity-flow drain line or condensate pump, which pumps the water to a floor drain or outside the home via a drain line.
Clean or Change the Filter
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.
Check the Condensate Pan
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.
Turn off the air conditioner at the main breaker box. Put on gloves and safety goggles, then open the cover, if possible, and wipe out the condensate pan. Clean the pan drain by using a piece of wire, drain-cleaning brush or plumber's snake to clean out the drain line opening. Then, go outside and attach a wet/dry shop vacuum or manual pump to the drain line and suck out clogs, dirt and debris.
Practice Preventive Maintenance
After cleaning the condensate pan and drain line, practice regular maintenance, including using a condensate pan cleaner according to the package directions. Alternately, pour 1/4 cup household vinegar or hydrogen peroxide into the drain pan monthly to help clean the pan and drain line. Allow to sit for 30 to 60 minutes, then rinse with a gallon or two of warm water. Avoid using chlorine bleach, as it corrodes the metal surfaces of the evaporator coils and interior of the air conditioner.
Call an HVAC Technician
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 types of mold and mildew are quite damaging to indoor air quality. If you suspect your ducts to be the issue, contact an HVAC technician trained in mold removal to clean and/or replace the ducts.