Mini-Split Heat Pump “Dirty Sock Syndrome”: What It Is; How to Prevent It.

Dirty sock smell will annually manifest itself when the season changes from COOLING to HEATING. Upon a visual inspection, the indoor coil space appears to be free of obvious surface mold and bacteria. In reality, however, any structure with an HVAC system can harbor dangerous hidden mold and bacteria which easily find a refuge within the indoor coil-those producing the dirty sock like odor.

Indoor HVAC coils, especially higher-efficiency coils, are prone to mold and bacteria growth because of the way they are designed and the fact that they intake large amounts of bacteria-laden indoor air.

The Science Behind It

“Cooling coils and other a/c components are not made in sterile environments, meaning that microbes cover all of their parts along with a small amount of lubricant. During the first cooling event after installation, condensate brings life to the microbes, which initially use the lubricant as an initial food source. Recirculated air contains a variety of organic materials and microbial species that aide ongoing decay and microbial proliferation.”

The cycle of microbial growth in HVAC systems also tends to be self-perpetuating; micro-organisms have short lifespans, and as such, the initial generation of bacteria inhabiting the HVAC system soon leaves a pile of miniscule carcasses which provide food for the next generation. The more generations there are, the more food there is, and the bigger the colony of mold-causing bacteria becomes!

High-efficiency systems are particularly prone to Dirty Sock Syndrome because, as the surface area of the coil becomes more compact (as is the case with mini-split higher efficiency coils), the coil fin spacing gets progressively tighter (there are more fins per inch of coil). The depth of the coil may also increase due to the fact that manufacturers want to expose a greater amount of coil surface to the air stream in order to improve coil heat exchange (and in the process improve system efficiency). While this is indeed effective in terms of maximizing efficiency, as the coils grow in size, depth, and fin density, they become even more likely to trap bacteria and mold.

Prevention and Treatment

If you detect the tell-tale organic, musty odor, have a HVAC professional clean the evaporator coil thoroughly with a non-acid coil cleaner and completely clean out the drain pan. The manufacturer also suggests a UV Treatment of the factory option pre-filter.

To learn more about mini-split heat pumps, reach out to the ductless experts at Geary Pacific Supply.