What is the White (or Grey) Slimy Gunk Coming from My A/C Drainline?

Bacteria (white slime):

Bacteria (it was previously commly thought to be algae, but is now known to be bacteria) seeks a warm/moist environment to grow with a good food supply, which can be found in oversized, short cycling AC equipment as well as dirty air filters supplying their food. A symptom of bacteria run amuck is your a/c system not cooling. Bacteria grows and it travels up your condensate drain line into your air handler unit. This bacteria clings to the side of the drain pipe and starts to grow in long, slimy tendrils. Water continues to travel down the piping to the outdoors, continually pushing on the bacteria. after the bacteria has grown for a while, the moving water can break off a section of bacteria (or even the leading edge of a long tendril) and that moving bacteria colony chunk can lodge in a narrowed section of drainline where other bacteria is growing. At this point the AC system shuts down or floods water from the indoor equipment.

Service Procedure:

When our technicians visit your home for a unit not cooling in the summer months, the first thing they inspect is the condensate removal system. Float switches in drain overflow lines or on emergency drain pans, are designed to shut down an air conditioner and prevent water damage. After finding water at a float switch, our technician will clear or vacuum the water at the indoor ac unit or pan. Next, the tech will vacuum the drain line from the outside of the home. An optional treatment for very stuck drains is a chemical cleaning. The technician carefully putting a drain clearing chemical into the drainline and following up with clean water to complete the flush. This repair can last many months, but will never be permanent. At Advanced Cooling we have had much success with replacing problem drain lines with larger piping to give the bacteria a hard time plugging the larger area.

Preventive Maintenance:

A handy homeowner can perform a monthly filter change and wet vacuuming during the summer. The idea behind this procedure is preventing the drain line from plugging up in the first place so the air conditioner never shuts off and a service call is not needed. The filter change will prevent dirt (food) from getting to the bacteria to feed it. After you find the condensate drain pipe exit which is a white pipe typically near the outdoor air conditioner section. Vacuum the put with a wet vacuum for 5 minutes or so making sure to form a tight seal from the vacuum hose to the drain pipe. When you are completing the procedure make sure it sounds like nothing more is coming out (the vacuum hose tends to shudder when it is removing gunk). Pro Tip: if you change your filter monthly, vacuum the drain line on the same day.

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