From information gathered over many years, most professionals agree the “Dirty Sock Syndrome” is caused 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 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. Heat pumps and air conditioners 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.