By David Kottke of Fox Service Company

Do you get a mildew-type odor when your HVAC system first starts up?  If you do, here is why:

From information gathered over many years, most professionals agree  “Dirty Sock Syndrome” is caused by a 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. Read more. →