A common service call that arrives in the latter days of summer is a clogged drain line. It is par for the course in the Southeastern United States; we have warm temperatures combined with high humidity. Air conditioning systems work to address both the temperature of the home (sensible heat) as well as the humidity of the home (latent heat). As the HVAC system runs it absorbs both heat and humidity from the home into the evaporator section of the unit. The heat is transferred outside via the refrigerant lines and superheated refrigerant while the humidity is transferred outside the home via the HVAC system drain line.
Slime in a drain is a universal problem. It occurs in shower drains, sink drains, and even appliance drains. There are many variables that can increase the likelihood and frequency in slime forming in the HVAC drain system. However, two key ingredients are always present: food and moisture. Food is provided by dust or dirt that manages to get into the HVAC system. This dust and/or dirt will contain some organic matter which will serve as a food source. This will combine with already present moisture from normal cooling operations and provide a moist environment with a food source. The result will be a slimy gunk that will easily clog the HVAC drain line. The drain lines in an HVAC system are not under any pressure, therefore it does not take a large amount of resistance to block the drainage of condensate.