What’s That Smell?
It’s as if there’s a dirty sock in the vent

From information gathered over many years, most professionals agree the “Dirty Sock Syndrome” is cause by bacteria that collects and grows on the indoor coils of heat pumps and air conditioners. Complaints normally surface when heat pumps go into defrost or when systems are run in heating for a brief time then switched back into cooling. The bacteria or odor collects and grows on the coil during this time heating time and is released all at once when the indoor coil gets cool and damp. Heat pump owners notice the odor when the system goes into defrost, usually when the outdoor temperature dips below 40 degrees. Both type systems can also experience the problem when the air conditioner is turned back on after the heating has been used. Our climate can require heating in the morning and cooling in the afternoon. This is a perfect condition for a complaint to surface. Most customers describe the odor as a musky dirty locker room smell. The problem is similar to an odor that is blown out of a car air conditioner when the a/c has not been used for a while.

It is very important to properly identify the problem before any action can be taken to solve the complaint. Many odor problems are incorrectly labeled. Effort should be spent to eliminate dirty drain pans holding water, drain lines connected to plumbing systems without adequate traps or dry traps, and return air leaks in ductwork or chases. If the odor is present ALL the time, especially during heat, the problem is NOT a “dirty sock syndrome” complaint. In this type of problem it only smells when the indoor coil gets cool and the bacteria are released into the air stream.

The Dirty sock syndrome plagues 0.5% to 2% of heat pumps in the southern states of with Texas being the lower side of the percentage. The syndrome is not a brand specific, with all manufacturers acknowledging complaints. The problem itself is sporadic and limited to isolated households and is somehow related to living style or products in the home. This can be proven as Trane or other manufacturers have documented changing out systems with new product and the complaint returns. Much effort and expense has been given by this industry to research and solve this syndrome, Trane alone admits to spending tens of thousands of dollars in their effort to determine what could be in these homes that creates the complaints, Trane has taken numerous air samples looking for a common denominator with no successful results.

Once the problem has been properly identified, action can be then taken to resolve or reduce the complaint. Some homes will have a minor problem while others may be severe. A mild complaint may be described as one that has taken 3 winters to surface while a severe problem may be in its first winter.

The first action that should take place is a thorough cleaning of the evaporator coil with a non-acid coil cleaner. This cleaning will bring the system back to normal and solve the problem.

If the coil cleaning does not resolve the complaint to a satisfactory level, next suggested action would be to clean the coil again and apply a coating of antifouling spray. Antifouling spray is designated to continuously protect coils from odors caused by airborne contaminates. It will take about 30 minutes for the spray to work before putting the system back online.
As a last resort, a specially coated replacement coil can be installed. This special phenolic coating is usually used to prevent corrosions on coils in harsh environments but has been found to reduce odor complaints on these types of systems. Only new coils can be coated, so the cost of a new coil and the coating is involved. The company that can provide this coating is AME (Aero-Marine Engineering, Inc.) and the coating is called TechniCoat 10-1. AME is at 1-800-874-4543.

*Revised edition by Hallmark from INCE Distributing, San Antonio TX 78230. 07/19/2002.