How do I stop my air conditioner from getting slime?

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.

In areas with excessive moisture through high humidity, the growth of this slime can be far more prevalent … talking about you, Southeast United States! In addition, oversized equipment that short cycles regularly will increase the growth rate of slime due to the constant down cycles where the inside of the HVAC system is damp and somewhat warm. This creates a much more accommodating growth environment for slime. Properly sized equipment with long run cycles will reduce the growth rate of slime. In addition, well run drain lines will aid in keeping internal water levels lower, which can slow or even limit the growth ability of slime in a drain pan. When considering the no-pressure drain system of an air conditioner, simple steps like ensuring a deep drain trap on negative pressure systems (like heat pumps) or a vent placed before the drain trap on positive pressure systems (like a cased coil in front of furnace) can make a large difference in the water level maintained in the internal drain pan of the system. Less water will equal less slime.

On the subject of slime forming in newer systems, units roughly 3-6 months old, there seems to be an increase in frequency. This creates some frustration on the part of contractors when dealing with a drain clog on a newer system. To be clear, the clog is still the result of slime which is the result of fungus/algae growth. The OEM manufacturer (regardless of which one) is not placing slime in new units during the manufacturing process. However, there is some credibility to the thought that oils used during the building of evaporator coils may serve as an inadvertent food source for the micro-organisms forming this slime until they have been washed off through normal condensation forming as part of the cooling cycle. It has not been determined how credible this theory is, but assuming some of these oils may be organic … there is some merit to the thought. This thought is further aided by the fact that many evaporators in the industry have transitioned from copper to aluminum tubing over the last 10-15 years. Copper itself has excellent anti-microbial properties that do not equally extended to aluminum. To date, no conclusive evidence has been presented to support this theory.

In dealing with slime formed in an HVAC system the first step is to thoroughly remove the slime and fully flush the drain system. Hot water is an excellent choice for flushing the pan and lines. Be very mindful to not use any corrosive chemicals in the flushing of the system as these chemicals could damage the evaporator tubing. The next step is to prevent this from forming again. Some popular prevention methods include anti-algae drain tablets or anti-algae drain pads. Another popular method in recent years has been to insert a small (3”to 4”) piece of 15% silver brazing rod in the primary drain pan; the silver will act as a natural anti-microbial material and help prevent growth. Other methods of maintaining a clean equipment environment include better air filtration to eliminate dust/dirt or the addition of Germicidal UV Lights; these methods will also have the added benefit of improving indoor air quality. At the end of the day our goal is to provide conditioned air to the homeowner. Conditioning is about much more than just cold air. We should also care about the cleanliness of the air which will extend to the cleanliness of everything the air comes across … drain pans included.

Matt is our Director of Technical Services and has been with East Coast Metal Distributors since March 2016. He is devoted husband and father to three boys in the Charlotte, NC area. He stumbled into HVAC completely by accident and is now climbing within the industry to be the best he can be. View all posts by Matt Rutland